Klinefelter Syndrome (XXY Syndrome)

Medically Reviewed by Minesh Khatri, MD on September 11, 2023
3 min read

Klinefelter syndrome is a genetic condition in which a boy is born with an extra X chromosome. Instead of the typical XY chromosomes in men, they have XXY, so this condition is sometimes called XXY syndrome.

Men with Klinefelter usually don’t know they have it until they run into problems trying to have a child. There’s no cure, but it can be treated.

You get the extra X chromosome by chance. Either the egg or the sperm that came together to create you had an extra X chromosome. Older women have a slightly higher chance of having a boy with XXY syndrome, but the chance is small.

Men with Klinefelter 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, in which you don’t have as many symptoms
  • More than one extra X chromosome, which is very rare and more severe

Some men show symptoms of Klinefelter as children, but others don’t know they have it until puberty or adulthood. Many men never realize that they have it because symptoms aren’t always present.

Symptoms of Klinefelter vary with age and include:


  • Hernia
  • More quiet than usual
  • Slower to learn to sit up, crawl, and talk
  • Testicles that haven’t dropped into the scrotum
  • Weaker muscles


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


  • 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
  • Puberty never comes, comes later, or doesn’t quite finish
  • Small penis and small, firm testicles
  • Taller than usual for the family


To diagnose Klinefelter syndrome, your doctor will start with a physical exam and questions about your symptoms and general health. They’ll probably 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 blood test looks at your chromosomes.
  • Hormone tests. These check hormone levels in your blood or urine.

It’s never too late to treat Klinefelter, but the earlier you start, the better.

One common treatment is testosterone replacement therapy. It can start at puberty and can spur 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 include:

  • Counseling and support for mental health issues
  • Fertility treatment (in some cases, using your own sperm to father a child)
  • Occupational therapy 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, things that might help include:

  • Playing sports and other physical activities to build muscles
  • Taking part in group activities to learn social skills

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

Treatment can help boys and men with Klinefelter live happy, healthy lives. Advances in fertility treatments have made it possible for some to father children. In general, life expectancy is normal. Some research has found that life expectancy for men with the condition may be a year or two less than those without it  because of other health problems linked to Klinefelter.