Health Topic

Klinefelter Syndrome

Klinefelter syndrome (KS) is a condition that occurs in men who have an extra X chromosome. Read about the common symptoms and treatments.

Reviewed Reviewed by a registered pharmacistGeneral information

Also called: XXY male

Important: This page is general information, not medical advice. It does not recommend or prescribe any treatment. Always consult your doctor or pharmacist before starting, stopping or changing any medicine.

About Klinefelter Syndrome

Klinefelter syndrome (KS) is a genetic condition that happens when a male is born with an extra copy of the X chromosome. KS can affect different stages of physical, language, and social development. It also usually causes infertility.

KS is not inherited. It's caused by a random error that happens when a sperm or egg is formed. This error causes a male to be born with an extra X chromosome.

Chromosomes are tiny "packages" in your cells that contain your genes. Genes carry information, called DNA, that controls what you look like and how your body works. Normally, you're born with a set of 46 chromosomes in each cell, two of which are the sex chromosomes. Females usually have two X chromosomes (XX), and males usually have one X and one Y (XY). In KS, the male usually has two X chromosomes and one Y (XXY).

In rare cases, a male could have XY in some cells and XXY in other cells. This is called mosaic Klinefelter syndrome. Males with mosaic Klinefelter syndrome may have fewer symptoms, depending on the number of XY cells they have in their bodies and where those cells are located.

Some males with KS may have no symptoms or very mild symptoms. So they might not know that they have KS, or they might not get diagnosed with it right away. In other cases, the symptoms can be more severe.

Boys with KS may be taller than other boys their age, with more fat around the belly. After puberty, they may have:

  • Smaller testes and penis
  • Breast growth (called gynecomastia)
  • Less facial and body hair
  • Reduced muscle tone
  • Narrower shoulders and wider hips
  • Weaker bones
  • Decreased sexual interest
  • Lower energy

Many of these symptoms happen because of low testosterone in the body. Testosterone is the main male sex hormone.

Boys with KS may also have learning or language problems. These problems may affect them socially, so they may be shy and quiet and can have trouble fitting in.

Most males with KS are infertile because they make little or no sperm.

There is no cure for KS, but treatments are available. The sooner the treatment is started, the better. If treatment is started by early puberty, it will likely help in reducing the symptoms.

Treatments for KS may include:

  • Testosterone replacement therapy
  • Surgery to remove or reduce breasts
  • Physical, speech, behavioral, and occupational therapy

In some cases, fertility treatments may help men with KS father children.

It sits within Genetics/Birth Defects and Men.

The lead U.S. institute for this subject is the Eunice Kennedy Shriver National Institute of Child Health and Human Development.

When to speak to someone. Reading about a topic cannot tell you whether it applies to you. Speak to a doctor or pharmacist if symptoms are severe, sudden, getting worse, or simply not going away — and seek emergency care immediately for chest pain, breathing difficulty, sudden weakness or confusion, or a serious allergic reaction.

Research on Klinefelter Syndrome

46 studies areregistered for this condition on ClinicalTrials.gov, the U.S. National Library of Medicine's public registry. A selection is below — the links go straight to the registry entry, not to a summary of ours.

What a registered study does and does not mean. Registration means a study exists and has been declared publicly. It says nothing about whether the treatment being tested works, is safe, or is available to you. Many studies find no benefit — that is what research is for. Never seek out an experimental treatment on the strength of a registry entry alone.

Research currently under way

Research already completed

Study data from ClinicalTrials.gov, a service of the U.S. National Library of Medicine. Retrieved 2026-07-30. Registry entries are supplied by study sponsors and investigators; ClinicalTrials.gov does not verify their scientific validity. PocketsInfo is not affiliated with, and not endorsed by, the NLM or the NIH.

Where to read more

Rather than paraphrase, we point you to the primary sources. Every link below is to a government health agency, a national institute or a recognised medical body — grouped by what you are trying to find out.

Learn More

Genetics

Clinical Trials

Medical subject headings

Clinicians and researchers index this subject under Klinefelter Syndrome. These are the terms to use when searching medical literature.

This page is built from the MedlinePlus health topic record for Klinefelter Syndrome, a service of the U.S. National Library of Medicine (NLM), National Institutes of Health. Information is from MedlinePlus.gov. PocketsInfo is not affiliated with, and is not endorsed by, the NLM or the NIH. Retrieved 30 July 2026.