Merck Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Klinefelter Syndrome

(Klinefelter's Syndrome; XXY)


Nina N. Powell-Hamilton

, MD, Sidney Kimmel Medical College at Thomas Jefferson University

Reviewed/Revised Nov 2023
Topic Resources
  • Klinefelter syndrome occurs when a boy has one extra X chromosome.

  • Boys may have learning disabilities, long arms and legs, small testes, and sometimes enlarged breasts.

  • The diagnosis is suspected at puberty when most of the symptoms develop.

  • Treatment with testosterone may be of benefit to some people.

The sex chromosomes Sex chromosomes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or the code for functional ribonucleic... read more Sex chromosomes determine whether a fetus becomes male or female. A pair of X and Y chromosomes (XY) results in a male, and a pair of X and X chromosomes (XX) results in a female.

Klinefelter syndrome is the most common sex chromosome disorder. Most boys inherit the extra X chromosome from their mother.

Symptoms of Klinefelter Syndrome

Most boys with Klinefelter syndrome have normal or slightly decreased intelligence. Many have speech and reading disabilities and difficulties with planning. Most have problems with language skills. Lack of insight, poor judgment, and impaired ability to learn from previous mistakes often cause these children to get into trouble.

Although their physical characteristics can vary greatly, most are tall with long arms and legs. They may also have wider hips but otherwise have a relatively normal appearance.

Puberty usually occurs at the normal time, but the testes remain small. At puberty, growth of facial hair is often sparse, and, for some boys, the breasts may enlarge somewhat (gynecomastia Breast enlargement in men Breast disorders occur infrequently in men. Breast disorders include Breast enlargement Breast cancer Breast enlargement in males is called either gynecomastia or pseudogynecomastia. Gynecomastia... read more ). Males with the syndrome are often infertile, but sometimes the testes develop enough to produce sperm.

Some affected boys have 3, 4, and even 5 X chromosomes along with the Y. As the number of X chromosomes increases, the severity of intellectual disability and physical abnormalities also increases. Each extra X is associated with a 15- to 16-point reduction in intelligence quotient (IQ), with language most affected, particularly expressive language skills.

Diagnosis of Klinefelter Syndrome

  • Before birth, testing of the mother

  • After birth, blood test

After birth, the syndrome is usually first suspected at puberty, when most of the symptoms develop. Analysis of the chromosomes using a blood test confirms the diagnosis of Klinefelter syndrome. However, many men are diagnosed during an infertility assessment.

Treatment of Klinefelter Syndrome

  • For children, speech and language therapy

  • For adolescents, testosterone hormone replacement therapy for life

Boys with Klinefelter syndrome usually benefit from speech and language therapy and eventually can do well in school.

Males with Klinefelter syndrome may require supplemental testosterone beginning at puberty. The hormone improves muscle bulk; bone density, making fractures less likely; and stimulates development of a more masculine appearance. Hormone replacement therapy may help lessen some behavior and developmental problems.

More recently, fertility preservation counseling has become an important part of the care of adolescents with Klinefelter syndrome. Obtaining and preserving sperm cells from males who produce them can give them the opportunity to father biological children.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Androderm, AndroGel, Andro-L.A., Aveed, AXIRON, Delatestryl, Depo-Testosterone, FORTESTA, JATENZO, KYZATREX, Natesto, STRIANT, Testim, Testoderm, Testopel, TLANDO, Virilon, Vogelxo, XYOSTED
quiz link

Test your knowledge

Take a Quiz!