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Klinefelter's syndrome

Klinefelter's syndrome is a genetic condition that only affects males. Affected males have an extra X chromosome. It is sometimes just called XXY. Between 1 in 500 and 1 in 1,000 boys are born with Klinefelter's syndrome (KS). Many people with KS are not diagnosed until they are adults, and it is believed that up to two thirds may never be diagnosed at all.

Males with Klinefelter's syndrome have small testicles (testes) which do not produce enough of the male hormone testosterone before birth and during puberty. This lack of testosterone means that during puberty, the normal male sexual characteristics may not develop fully. There tends to be reduced facial and body hair, and some breast tissue often develops. The lack of testosterone is also responsible for other symptoms of Klinefelter's syndrome, including infertility.

Treatment includes testosterone replacement. This can help improve some of the symptoms but does not have an effect on fertility. The majority of men with Klinefelter's syndrome lead an independent life, forming relationships and getting a normal job. They also often have a normal lifespan.

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What is Klinefelter's syndrome (KS)

In general, each cell in your body contains 46 chromosomes arranged in 23 pairs. One chromosome from each pair is inherited from your mother and the other is inherited from your father. One of these chromosome pairs is known as the sex chromosomes because this pair of chromosomes determines our sex.

Females have two of the same kind of sex chromosome (XX). Males have two different sex chromosomes (XY). The Y chromosome contains the male determining genes. So a female normally has 46, XX chromosomes and a male normally has 46, XY.

In KS, males have an extra X chromosome. It is a genetic problem that only affects boys and men. It is a sex chromosome trisomy. Instead of being 46, XY, men or boys with KS are usually 47, XXY.

Dr Harry F. Klinefelter first described this syndrome in the USA in 1942. The condition affects sexual development. Males with KS have small testicles (testes) which do not produce enough of the male hormone testosterone before birth and during puberty.

This lack of testosterone means that during puberty, the normal male sexual characteristics do not develop fully. There is reduced facial and pubic hair, and some breast tissue may develop. The lack of testosterone is also responsible for some of the other symptoms (see above), including infertility.

Klinefelter's syndrome symptoms

KS is usually not noticed until you go through puberty. Puberty may be late or incomplete if you have KS. Sometimes the condition may only be diagnosed if you are investigated for infertility as an adult. Features are often subtle and may not be noticed or may not be a problem. Not all people with KS have all the typical features.

The typical features of KS in an adult are:

  • Small testicles (testes). This feature is present in most people with KS.

  • Decreased facial hair compared to a usual male.

  • Some breast tissue development (gynaecomastia).

  • Decreased pubic hair compared to a usual male.

  • A tall, thin body with disproportionately long arms and legs.

  • Obesity - this may cause a thicker waistline but it is often partially disguised by wider-than-normal hips.

Children and adolescents with KS may also have:

  • A delay in the age of motor milestones eg, when they sit up, crawl and walk.

  • A condition which causes co-ordination problems (dyspraxia).

  • Speech and language delay or difficulties.

  • Attention problems.

  • Mild learning disabilities.

  • A condition which causes problems with reading, writing or spelling (dyslexia).

  • Behavioural problems - boys tend to be shy and have low self-confidence; they may appear to be immature when compared to their peers.

In adulthood, in addition to the main typical features:

  • There may be problems getting an erection, and a low sex drive (low libido).

  • Anxiety and depression may be a problem.

  • 'Thinning' of the bones (osteoporosis) may develop in young or middle age rather than the usual older age for this condition.

  • A physique which is less muscular than other men.

  • Most men who are 47, XXY have normal intelligence. However, intelligence may be affected.

  • Almost all men who are 47, XXY will be infertile (sterile). That is they will not be able to father a baby without medical help.

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Causes of Klinefelter's syndrome

KS is a genetic condition but it is not 'inherited' from your parents. It usually happens randomly during meiosis. Meiosis is cell division which produces egg and sperm cells (reproductive cells).

Because of an error in cell division called nondisjunction, an egg or a sperm cell can have an extra copy of an X chromosome. This means that when it meets a 'normal' egg or sperm at conception, the embryo produced (which will go on to develop into the baby) will have one extra copy of the X chromosome in each of their body's cells. This is known as a sex chromosome trisomy (there are three sex chromosomes in the body's cells instead of two).

Mosaic Klinefelter's syndrome also occurs randomly and is not 'inherited'. During early development in the womb (uterus), there is a problem with cell division. This means that some of the body's cells have one X chromosome and one Y chromosome (46, XY), and other cells have an extra copy of the X chromosome (47, XXY).

It is thought that mothers or fathers who are at an older age may be more likely to have a child with KS.

How is Klinefelter's syndrome diagnosed?

KS is often diagnosed when a man is being investigated for infertility. The diagnosis can be confirmed by a test called a karyotype. A blood sample is taken from the man and the chromosomes are studied. This test can show the extra X chromosome.

Sometimes KS is diagnosed in an unborn baby boy during antenatal screening carried out for another reason. If, for example, screening tests have shown an increased risk of Down's syndrome, the mother may go on to have a further test called an amniocentesis. In this test, a sample is taken from the amniotic fluid around the baby in the womb (uterus). A karyotype test can then be done on the sample and can show if the baby has KS.

KS may also be diagnosed at the time of puberty. Puberty may be delayed or noticed as abnormal. A doctor may notice signs of KS when they examine you; for example, if your testicles (testes) are small, or if you have breast tissue development (gynaecomastia) or reduced pubic hair for your age. Your doctor may then suggest a karyotype blood test.

Other blood tests may also be done if your doctor suspects that you may have KS. These include hormone blood tests. For example, if you have KS, towards the end of puberty and in adulthood your levels of testosterone in your blood will be low. Other hormone levels - luteinising hormone and follicle-stimulating hormone - tend to be higher than normal.

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Klinefelter's syndrome treatment

If you have KS, you will usually be referred to a hormone specialist (an endocrinologist).

Testosterone replacement

The main treatment for KS is with testosterone to boost the low levels. Ideally, testosterone replacement should begin at puberty and is needed lifelong. Testosterone can help to:

  • Increase your strength and build a more muscular body type.

  • Increase facial and pubic hair growth.

  • Increase your libido (sex drive).

  • Enlarge your testes.

  • Improve your mood and self-esteem.

  • Protect against 'thin bones' (osteoporosis).

  • Reduce the risk of cardiovascular disease.

However, testosterone treatment does not have any effect on your fertility. It also has little effect on breast tissue development (gynaecomastia).

Treatment for gynaecomastia

Gynaecomastia can be a troubling problem for males with KS. Sometimes men choose to have surgery to remove the breast tissue.

Infertility treatment

If you have KS and you are considering having children, you will need to see an infertility specialist at an assisted reproductive health clinic. There have been great developments in the treatment of infertility over recent years and there have been cases of men with KS fathering children. Also, men with variants of KS can be fertile (mosaic KS - see the section called 'Klinefelter's syndrome details and causes' below).

Investigation will show if you have any viable sperm in your testes. If this is the case, the sperm may be able to be extracted from your testes. The sperm may then be used for in vitro fertilisation (IVF) or intra-cytoplasmic sperm injection (ICSI). ICSI involves an individual sperm being injected directly into an egg. The egg containing the sperm is then placed in the womb in the same way as with IVF. See the separate leaflet called Infertility Treatments for more details.

However, you may wish to see a genetic counsellor first. The exact risk of someone with KS having a child with KS or another chromosomal problem is unknown, but it appears to be low.

Other treatments

Other treatments for KS may include, for example, speech and language therapy and behavioural therapy in children (if required). Physiotherapy and occupational therapy are also sometimes needed.


Males with KS may have an increased risk of certain conditions affecting other body systems, including:

What is the outlook (prognosis) for Klinefelter's syndrome?

Sometimes boys with KS may struggle through adolescence. They may have difficulties at school, many frustrations and, occasionally, may have serious emotional or behavioural difficulties.

However, the majority of men with KS go on to lead an independent life, forming relationships and getting a normal job.

In itself, KS does not shorten lifespan. However, life expectancy may be reduced if one of the conditions above develops (such as diabetes and/or cardiovascular disease), as these can reduce lifespan.

Further reading and references

Article history

The information on this page is written and peer reviewed by qualified clinicians.

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