The ABO blood-group distributions of 1460 patients who had died from a stroke were compared with those of a control group of 20 705 controls selected at random from the healthy population at risk (i.e., over thirty-five years of age and matched for age and sex ratio). The cause of death was certified as cerebral thrombosis in 329 cases and as cerebral haemorrhage in 482 cases, these diagnoses being established in neurological hospitals; the remaining 649 cases had an unspecified type of stroke, the diagnosis being made by general practitioners. In the group with unspecified type of stroke the blood-group distribution was practically the same as the distribution in the controls. In the thrombosis cases there was an excess of blood-groups A and AB and a deficiency of O and B; in cerebral haemorrhage this situation was reversed. However, these were only trends; the differences were not significant at the 5% level. A statistically significant difference did emerge when the A+AB excess in thrombosis was contrasted with the O+B excess in haemorrhage, suggesting that this difference might be accounted for the major A subgroup (A1) and, consequently, A1B.