The original syndrome described by Klinefelter consisted of GYNAECOMASTIA, testicular ATROPHY and INFERTILITY. Some patients have associated learning disability and striking tallness of stature. A small penis, loss of body hair and a high-pitched voice may be present.

Most patients with Klinefelter's syndrome have 47 chromosomes instead of the normal 46. The extra chromosome is an X chromosome, so that the sex chromosome constitution is XXY instead of XY. Klinefelter's syndrome is one of the most common chromosome abnormalities, and occurs in 1 in 300 of the male population.