Klinefelter syndrome is the most common of all sex chromosome abnormalities, and is caused by the presence of one or more additional X chromosomes.

Klinefelter syndrome is seldom seen before puberty, at which time varying degrees of failure of adolescent virilization (the biological development of sex differences) occur.

All males with Klinefelter syndrome have:

- azoospermia (the absence of sperm in the semen)
- small testes
- defective development of secondary sex characteristics

Some males may also have:

- cognitive impairment
- gross motor skill difficulties
- developmental language delay
- poor verbal skills
- reduced auditory memory
- shyness
- passivity
- behavioral problems
- school difficulties

Therapy for males with Klinefelter syndrome is directed at enhancing the masculine characteristics through administration of testosterone.

Klinefelter syndrome is named after Dr. Harry Klinefelter, an endocrinologist at Massachusetts General Hospital, Boston, Massachusetts, who first described it in 1942.