Klinefelter syndrome

Klinefelter syndrome was first described in 1942 by Dr Harry Klinefelter and colleagues. It is a chromosomal abnormality that only occurs in 1 in 500 to 1,000 newborn boys. A female usually has an XX pair of sex chromosomes and a male an XY pair. Klinefelter syndrome occurs due to an extra copy of the X chromosome in males.

In this article

What are the symptoms of Klinefelter syndrome?

The most common symptom is infertility. Other symptoms include abnormal body proportions (long legs, short trunk, shoulder equal to hip size), less than normal amount of pubic armpit and facial hair, small, firm testicles, reduced muscle and tall stature. Breast development may appear after puberty in 25 per cent of those affected. In some cases, intellectual difficulties, especially in verbal skills, may be experienced. Additionally psychological problems with self-image may be experienced by adolescent boys and adult men.

Other chromosome abnormalities involving more than two X chromosomes in males may lead to more severe learning difficulties.

What are the causes of Klinefelter syndrome?

Males affected by Klinefelter syndrome have two X chromosomes, as well as one Y, resulting in the formation XXY. A mosaic form also occurs where only a percentage of body cells contain XXY while the remainder carry XY. The extent of the affect will depend upon the proportion of XXY to XY throughout the body. Males with Klinefelter syndrome typically have small testes that do not produce enough testosterone, which is the hormone that directs male sexual development before birth and during puberty.

How is Klinefelter syndrome diagnosed?

Klinefelter syndrome may first be diagnosed when a man comes to the doctor because of infertility, as it is the most common symptom. Examining sex chromosomes to determine number (karyotyping) can confirm the diagnosis. It may also be necessary to test hormone levels, including estradiol (a type of oestrogen), follicle stimulating hormone, luteinising hormone and testosterone.

How is Klinefelter syndrome treated?

Boys diagnosed with Klinefelter syndrome may benefit from speech and language therapy and other support to learn to express themselves. They may also benefit from educational support and occupational therapy to increase muscle tone. Testosterone therapy may be prescribed to help grow body hair, improve appearance of muscles, improve mood and self-esteem, increase energy and sex drive and increase strength.

Most men with this syndrome will be unfertile so referral to a fertility specialists for advice may be required. In some cases, surgical microdisection of the testes may reveal sperm and allow fertility using intracytopalsmic sperm inoculation or of an egg and in vitro fertilisation.

More advice from Contact

Early years support

From diagnosis and common concerns to childcare and early years education, we’re here for you and your child.

early years support

Inheritance patterns and prenatal diagnosis

Inheritance patterns
Klinefelter syndrome usually occurs as a sporadic event. Women who get pregnant after age 35 years are slightly more likely to have a boy with this syndrome than younger women.

Prenatal diagnosis
Chorionic villus sampling at 10 to 12 weeks or amniocentesis at about 16 weeks is available to older mothers to determine if the child is at risk of Klinefelter syndrome.

Is there support for people affected by Klinefelter syndrome and their families?

If your child is affected by a medical condition or disability, we can help. Call our freephone helpline on 0808 808 3555 to get information, support and advice. We also offer emotional support for parents via our Listening Ear service.

We have a range of parent guides on aspects of caring for a disabled child in our resource library. You may also find our Early Years Support useful, which contains links to parent carer workshops and help for families going through the diagnosis process.

We’ve listed some support groups below and you can also meet other parents online in our closed Facebook group.

Klinefelter Syndrome Association (KSA)

Tel: 0300 111 4748
Email: Vicechair@ksa-uk.net
Website: ksa-uk.net

The Association is a Registered Charity in England and Wales No. 1058319. It provides information and support to all affected by, or having an interest in, Klinefelter’s Syndrome. The Association holds an annual conference and AGM in June and runs an activity weekend in spring and autumn. 

Group details last reviewed May 2023.


Medical text written November 1991 by Contact a Family. Approved November 1991 by Professor M Patton, Professor of Medical Genetics, St George’s Hospital Medical School, London UK and Dr JE Wraith, Consultant Paediatrician, Royal Manchester Children’s Hospital, Manchester, UK. Last updated March 2012 by Dr R Stanhope, Consultant Paediatric Endocrinologist, The Portland Hospital, London UK.

Back to A-Z Conditions