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

Symptoms, when present, vary from one person 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 have infertility. Other symptoms may include delayed speech development, poor verbal skills, intellectual disability, or emotional and behavioural 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 therapy to increase testosterone. Other treatments vary depending on the symptoms and may include educational support for language and learning difficulties and counselling or social skills training for behaviour problems.