Mortality and cancer incidence were assessed in a cohort of 1373 patients with numerical sex chromosome abnormalities diagnosed at three cytogenetics centres in Britain during 1959–90, and were compared with expectations from national rates. Four hundred patients with Turner's syndrome were followed, of whom 62 died, with a relative risk (RR) of death of 4.16 (95% confidence interval (CI) 3.22–5.39). Turner's syndrome patients had greatly raised risks of death from diseases of the nervous, cardiovascular, respiratory, digestive and genitourinary systems. One hundred and sixty three deaths occurred among 646 patients with Klinefelter's syndrome with a 47,XXY constitution, giving an RR of 1.63 (1.40–1.91). Mortality in these patients was significantly raised from diabetes and diseases of the cardiovascular, respiratory and digestive systems. There was also significantly increased mortality for patients with X polysomy (RR = 2.11 (1.43–3.02)) and Y polysomy (RR = 1.90 (1.20–2.85)), the former with significantly increased mortality from cardiovascular disease and the latter from respiratory disease. The only significantly raised risks of cancer incidence or mortality in the cohort were for lung cancer and breast cancer in patients with Klinefelter's syndrome with a 47,XXY constitution, and non-Hodgkin's lymphoma in men with more than three sex chromosomes.