When described more than 40 years ago, Klinefelter's syndrome (small testes, sterility, increased excretion of follicle-stimulating hormone, and usually gynecomastia) was thought to be an endocrine disorder. A second testicular hormone was postulated but has never been isolated. During the ensuing years, the syndrome has been found to be a chromosomal disorder, in which there is an extra X chromosome in 80% of the patients. The disorder occurs once in 500 to 1,000 male births and is best diagnosed by a buccal smear. When there is androgen deficiency, it is treated with testosterone. Gynecomastia is treated surgically because of the potential danger of malignancy or for cosmetic reasons.