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Adouble-blind trial of aspirin for the treatment of cerebral ischemia was begun in 1972 and continued for 37 months. This was accomplished despite difficulties in controlling a long-term study of a drug which has widespread availability and consumption. The study design, criteria for selection of patients, follow-up surveillance, and methods of data analysis are presented. We report only subjects without carotid surgery before randomization. Patients (178) who had carotid transient ischemic attacks (TIAs) were randomly allocated to aspirin or placebo and followed to determine the incidence of subsequent TIAs,death, cerebral infarction or retinal infarction. Analysis of the first six months of follow-up revealed a statistically significant differential in favar of aspirin when death or cerebral or retinal infarction and the occurrence of TIAs were grouped and considered together as end points. Significance in favor of aspirin treatment was mainly revealed in patients with a history of multiple TIAs and was most evident in those individuals having carotid lesions appropriate to the TIA symptoms. It cannot be inferred from this study that aspirin prevents stroke because when end points were restriced to death or cerebral or retinal infarction, there was no statistically significant differential between the aspirin and placebo treatments.