Klinefelter syndrome (sometimes called Klinefelter's or XXY) is a condition affecting boys and men where they're born with an extra X chromosome.

Chromosomes are packages of genes found in every cell in the body. Two types of chromosome, called the sex chromosomes, determine the genetic sex of a baby. These are named either X or Y.

Usually, a female baby has two X chromosomes (XX) and a male has one X and one Y (XY). But in Klinefelter syndrome, a boy is born with an extra copy of the X chromosome (XXY).

Boys and men with Klinefelter syndrome are still genetically male, and often will not realise they have this extra chromosome, but occasionally it can cause problems that may require treatment.

Klinefelter syndrome is quite common, affecting around 1 in every 660 males.

This page covers:


Health issues


Testing and diagnosis


More information and support

Symptoms of Klinefelter syndrome

Klinefelter syndrome doesn't usually cause any obvious symptoms early in childhood, and even the later symptoms may be difficult to spot.

Most people with the condition don't realise they have it.

Possible symptoms include:

  • in babies and toddlers – learning to sit up, crawl, walk and talk later than usual, being weaker, quieter and more passive than normal
  • in childhood – shyness and low self-confidence, problems with reading, writing, spelling and paying attention, mild dyslexia and/or dyspraxia, low energy levels, difficulty socialising or expressing feelings
  • in teenagers – growing slightly taller than expected for the family (with long arms and legs), broad hips, poor muscle tone, reduced facial and body hair, a small penis and testicles, enlarged breasts (gynaecomastia)
  • in adulthood – inability to have children naturally (infertility) and a low sex drive, in addition to the physical characteristics mentioned above

Health issues in Klinefelter syndrome

Most boys and men with Klinefelter syndrome will not be significantly affected by the condition and can live normal, healthy lives.

Infertility tends to be the main problem, although there are treatments that can help (see Treatments for Klinefelter syndrome below).

However, people with Klinefelter syndrome are at a slightly increased risk of developing other health problems, including:

These problems can usually be treated if they do occur and testosterone replacement therapy may help reduce the risk of some of them.

Causes of Klinefelter syndrome

Klinefelter syndrome is caused by an additional X chromosome.

This chromosome carries extra copies of genes, which interfere with the development of the testicles and mean they produce less testosterone (male sex hormone) than usual.

The extra genetic information may either be carried in every cell in the body or it may only affect some cells (known as mosaic Klinefelter syndrome).

Klinefelter syndrome isn't inherited – it's caused by a problem with the formation of the sperm or egg that happens randomly. If you have a son with the condition, the chances of this happening again are very small.

The risk of having a son with Klinefelter syndrome may be slightly higher if the mother is over 35 years of age.

Testing for Klinefelter syndrome

See your GP if you have concerns about your son's development or you notice any troubling symptoms of Klinefelter syndrome in yourself or your child.

Klinefelter syndrome isn't necessarily anything serious, but treatment can help reduce some of the symptoms, if necessary.

In many cases, it's only detected if a man with the condition undergoes fertility tests.

Your GP may suspect Klinefelter syndrome after a physical examination and they may suggest sending off a sample of blood to be tested for unusual hormone levels.

The diagnosis can be confirmed by checking a sample of blood for the presence of the extra X chromosome.

Treatments for Klinefelter syndrome

There's no cure for Klinefelter syndrome, but some of the problems associated with the condition can be treated if necessary.

Possible treatments include:

Testosterone replacement therapy

Testosterone replacement therapy involves taking medication containing testosterone. It can be taken in the form of gels or patches, or given as injections.

It may be started after puberty begins and can help with the development of a deep voice, facial and body hair, and an increase in muscle mass.

Long-term treatment during adulthood may also help with several other problems associated with Klinefelter syndrome – including osteoporosis, low mood, reduced sex drive, low self-esteem and low energy levels – although it can't reverse infertility.

More information and support

If you or your son has been diagnosed with Klinefelter syndrome, you might find it useful to find out more about the condition and get in touch with others affected by it.

The following websites may be able to help:

Klinefelter's Syndrome Association UK

Klinefelter Organisation

Contact a Family

Information about your child

If your son has Klinefelter syndrome, your clinical team will pass information about him on to the National Congenital Anomaly and Rare Diseases Registration Service (NCARDRS)

This helps scientists look for better ways to prevent and treat this condition. You can opt out of the register at any time.

Page last reviewed: 23/06/2016

Next review due: 23/06/2019