Klinefelter syndrome

Key facts Klinefelter syndrome is a congenital (from birth) condition, where males are born with one or more extra X chromosomes. Young children with Klinefelter syndrome can have motor and language delay, as well as learning and behavioural problems. Adolescents and adults with Klinefelter syndrome may notice that they have a small penis and testicles, less facial and body hair and larger breasts than expected. Many people with Klinefelter syndrome don’t know they have it and have never received treatment. Testosterone therapy, usually started at puberty, can help with many of these features of Klinefelter syndrome. What is Klinefelter syndrome?