Twelve boys with Klinefelter's syndrome (47,XXY) identified by sex chromatin screening at birth were examined at between ages 16 and 18 years, together with 12 controls matched for social class and birth order from the same newborn population. Physical examination, psychometric assessment, personality, and degree of psychosexual development were assessed without knowledge of the karyotype. Anthropometry showed increased leg length and decreased head circumference in the XXY boys. Gynaecomastia was present in 4 boys, and testicular volume was reduced in the majority but one boy had normal sized testes. On the Wechsler intelligence scale there was a significant reduction in verbal score but not in either performance or full-scale score compared with the controls. Although appreciable differences were found in growth, personality, intelligence test scores, and psychosexual development, these were of small degree.