The causes of death in 466 X chromatin positive males (Klinefelter's syndrome) studied prospectively over the last 25 years have been analysed. We have previously reported the overall mortality to be increased by 50% and life expectancy reduced by about five years. A highly significant increase in mortality from cerebrovascular disease was observed in the sub group considered to be most representative of X chromatin positive males in general. In the age group up to 45 years this increase could be attributed to deaths from subarachnoid haemorrhage. An increase in mortality from respiratory diseases was observed in those ascertained in psychiatric hospitals. In the sample as a whole there were small but highly significant numbers of deaths from carcinoma of the breast and aortic valve disease. The deaths from carcinoma of the breast were comparable with those expected if female mortality rates were applied.