Cigarette smoking habit was studied in 621 patients with stroke and in 573 control subjects using case control methods. There was an excess of smokers among the stroke group when compared with control subjects. Relative risks of cigarette smokers compared with nonsmokers, after adjustment for the possible confounding variables for subarachnoid hemorrhage, intracerebral hemorrhage, and cerebral infarction, were 4.5, 1.8, and 3.2 for men and 2.5, 1.3, and 2.3 for women, respectively. For all subtypes of stroke combined, the increased relative risk was related to the daily intake of cigarettes, the heaviest smokers having a higher relative risk than that for light smokers. The estimated increase in relative risk among smokers was 1.5 for each 10 cigarettes smoked daily in both men and women. We conclude that cigarette smoke may be an important preventable factor for both hemorrhagic and nonhemorrhagic stroke.