What is Klinefelter Syndrome?

Everything from your height to your hair color goes back to your genes. They hold the code for how your body looks and works. Genes are bundled into chromosomes. One pair, called the sex chromosomes, determine whether you’re male or female.

Usually, females have two X chromosomes (XX). Males have an X and a Y (XY).

But in rare cases, a male is born with an extra X chromosome (XXY). This is Klinefelter syndrome. It’s also called Klinefelter’s or XXY.

Often, men don’t even know they have Klinefelter until they run into problems trying to have a child. There’s no cure, but it can be treated. With the right care, most men with Klinefelter lead normal, healthy lives.

What Causes It?

You get the extra X chromosome by chance. Either the egg or sperm that came together to make you just happened to have an extra X chromosome. Older moms have a slightly higher chance of having a boy with Klinefelter, but the increase in probability is very small.

You may have:

  • An extra X chromosome in every cell, which is the most common
  • An extra X chromosome in only some cells, called mosaic Klinefelter, where you don’t get as many symptoms
  • More than one extra X chromosome, which is very rare and more severe


Symptoms vary with age, and you don’t always get all of them. Some males show symptoms early on, but others don’t realize they have Klinefelter until puberty or into adulthood. And many men never even realize they have it.

Babies: They may have problems at birth, such as a hernia or testicles that haven’t dropped into the scrotum. You may see the following other signs in babies with Klinefelter:

  • More quiet than usual
  • Slower to learn to sit up, crawl, and talk
  • Weaker muscles

Children: Boys may have low energy levels or any of the following:

  • A hard time making friends and talking about feelings
  • Problems learning to read, write, and do math
  • Shyness and low confidence


Teenagers: During the teenage years, puberty may come on later, not quite finish, or not happen at all. Other possible symptoms include:

  • Larger breasts than normal
  • Less facial and body hair, and it comes in later
  • Less muscle tone, and muscles grow slower than usual
  • Longer arms and legs, wider hips, and a shorter torso than other boys their age
  • Small penis and small, firm testicles
  • Taller than usual for the family

Adults: In addition to the symptoms teenagers show, men may have:

Can It Lead To Other Conditions?

Many problems caused by Klinefelter are because of lower testosterone levels. You may have a slightly higher chance of:

Diagnosis and Tests

Your doctor will start with a physical exam and questions about your symptoms and general health. He’ll likely examine your chest, penis, and testicles and do a few simple tests, such as checking your reflexes.

Your doctor may then run two main tests:

Chromosome analysis: Also called karyotype analysis, this is a blood test that looks at your chromosomes.

Hormone tests: These check hormone levels in your blood or urine.


It’s never too late to get treatment, but the earlier you start, the better.

One common treatment is testosterone replacement therapy. It starts at puberty and can spur on typical body changes, such as facial hair and a deeper voice. It can also help with penis size and stronger muscles and bones, but it won’t affect testicle size or fertility.

Testosterone replacement therapy throughout your life can help prevent some of the long-term problems that come with Klinefelter.

Other treatments may include:

  • Counseling and support for mental health issues
  • Fertility treatment (in some cases, using your own sperm to father a child)
  • Occupational and physical therapy to help with coordination and build muscles
  • Plastic surgery to reduce breast size
  • Speech therapy for children
  • Support in school to help with social skills and learning delays

If your child has Klinefelter, you can suggest that he:

  • Play sports and other physical activities to build muscles
  • Take part in group activities to learn social skills
WebMD Medical Reference Reviewed by Carol DerSarkissian on December 28, 2018



Johns Hopkins Medicine: “Klinefelter Syndrome.”

NHS: “Klinefelter Syndrome.”

Mayo Clinic: “Klinefelter Syndrome.”

© 2018 WebMD, LLC. All rights reserved.