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Klinefelter syndrome is the presence of an extra X chromosome in a male.

Alternative Names:

47 X-X-Y syndrome

Causes, incidence, and risk factors:

Humans have 46 chromosomes. Chromosomes contain all of your genes and DNA, the building blocks of the body. The two sex chromosomes determine if you become a boy or a girl. Females normally have two of the same sex chromosomes, written as XX. Males normally have an X and a Y chromosome (written as XY).

Klinefelter syndrome is one of a group of sex chromosome problems. It results in males who have at least one extra X chromosome. Usually, this occurs due to one extra X (written as XXY).

Klinefelter syndrome is found in about 1 out of every 500 - 1,000 newborn males. Women who have pregnancies after age 35 are slightly more likely to have a boy with this syndrome than younger women.


The most common symptom is infertility . Other symptoms may include:

  • Abnormal body proportions (long legs, short trunk, shoulder equal to hip size)
  • Enlarged breasts (gynecomastia )
  • Sexual problems
  • Less than normal amount of pubic, armpit, and facial hair
  • Small, firm testicles
  • Small penis
  • Tall height

Signs and tests:

Adults may come to the doctor because of infertility . School-age children may be brought in to because of learning problems.

The following tests may be performed:


Testosterone therapy may be prescribed. This can help:

  • Grow body hair
  • Improve appearance of muscles
  • Improve concentration
  • Improve mood and self esteem
  • Increase energy and sex drive
  • Increase strength

Most men with this syndrome are not able to father children. However, some men have been able to have children. An infertility specialist may be able to help.

Support Groups: The American Association for Klinefelter Syndrome Information and Support (AAKSIS) -

Expectations (prognosis):

Most patients have a normal, productive life.


The syndrome increases the risk of:

Enlarged teeth with a thinning surface (taurodontism) is very common in Klinefelter syndrome. It can be diagnosed by dental x-rays.

Calling your health care provider:

Call for an appointment with your health care provider if a boy does not develop secondary sexual characteristics at puberty .

A genetics counselor can provide information about this condition, and help explain abnormal chromosome findings and possible complications. The counselor will also be familiar with local and national support groups.

An endocrinologist and infertility specialist may also be helpful.


Review Date: 10/15/2008
Reviewed By: Diana Chambers, MS, EdD, Certified Genetics Counselor (ABMG), Charter Member of the ABGC, University of Tennessee, Memphis, TN. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

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