A 4-year follow-up study was carried out on the prognosis of 54 patients (36 men and 18 women) who survived their first ischaemic brain infarction which had occurred under the age of 65. Twenty patients (15 men and 5 women) already had documented atherosclerotic vascular disease other than ischaemic brain infarction at the time of the hospital admission (atherosclerotic group). When both sexes were considered together, mortality during the 4-year follow-up was significantly higher in the atherosclerotic group than in the non-atherosclerotic group (6/20 versus 3/34; p less than 0.05) and this was due to an excess of cerebrovascular mortality (4/20 versus 0/34; p less than 0.05). The incidence of recurrent fatal or non-fatal ischaemic brain infarction was significantly higher in the atherosclerotic group than in the non-atherosclerotic group (8/20 versus 3/34; p less than 0.01). Among those patients who survived the 4-year follow-up period the incidence of new non-fatal atherosclerotic vascular events (cerebrovascular accident, myocardial infarction, other heart disease or intermittent claudication) was significantly higher in the atherosclerotic group than in the non-atherosclerotic group (8/14 versus 8/31; p less than 0.05). It is concluded, that the presence of atherosclerotic vascular disease at the time of first ischaemic brain infarction in patients under the age of 65 is associated with a significantly increased risk for recurrent ischaemic brain infarction or other new atherosclerotic vascular events.