May also be called: XXY Condition; XXY Syndrome

Klinefelter (KLINE-fell-ter) syndrome is a fairly common genetic condition found only in males that happens when a boy is born with an extra sex chromosome in most or all of his cells.

More to Know

Genetic material, or DNA, is contained in tiny string-like structures called chromosomes that are found inside the body's cells. A person's physical gender (whether they have male or female sex organs) is determined by the sex chromosomes: females have two X chromosomes, or XX; most males have one X chromosome and one Y chromosome, or XY. Males with Klinefelter syndrome are born with cells that have an extra X chromosome, or XXY. This can cause delayed development and puberty, a smaller penis and testicles, infertility, and other symptoms.

The condition that causes Klinefelter syndrome is present at birth and can't be changed. But medical and educational treatments and various types of therapy (physical, speech, behavioral, mental health, and occupational) can help boys have more normal puberty and physical development, keep pace in school, and overcome problems with shyness and social development.

Keep in Mind

Kids and teens with Klinefelter syndrome sometimes have trouble fitting in with kids their own age. With early intervention, in adulthood most will have normal social relationships with friends, family members, coworkers, and others.

All A to Z dictionary entries are regularly reviewed by KidsHealth medical experts.