Klinefelter syndrome is a genetic disorder that affects males. Normally, males have one X and one Y chromosome; males with Klinefelter syndrome have at least two X chromosomes and, in rare cases, as many as three or four.

Symptoms, when present, vary from one male to another depending on the number of extra X chromosomes and how many cells in the body are affected. Symptoms may include a lack of fully developed secondary male sexual characteristics—such as the growth of the testicles and penis, deeper voice, and body hair—by adulthood. These men usually cannot father children (are infertile). Other symptoms may include delayed speech development, poor verbal skills, below-average intelligence, or emotional and behavioral problems. Males with more than one extra X chromosome generally have more severe symptoms.

Klinefelter syndrome usually goes undiagnosed until the preteen years (around ages 11 to 12), when males often begin puberty. In some males, Klinefelter syndrome is never diagnosed and does not cause problems.

The main treatment for Klinefelter syndrome is hormone replacement therapy to increase testosterone. Other treatments vary depending on the symptoms and may include educational support for language and learning difficulties and counseling or social skills training for behavior problems.