Two hundred and three patients, 148 males and 55 females, who during the last month before admission had experienced at least one reversible cerebral ischemic attack of less than 72 hours duration, were randomly assigned to treatment with either acetylsalicylic acid (ASA) 1000 mg daily (101 patients) or placebo (102 patients). The average follow-up period was 25 months. The two treatment groups were comparable with respect to age, sex, associated diseases, risk factors, number and duration of cerebral ischemic attacks. No statistically significant differences were found between the treatment groups as to the primary end point: stroke or death (ASA group 20.8%, placebo group 16.7%). Occurrence of transient ischemic attacks during the treatment period was not reduced by ASA treatment, whereas there was a trend suggesting fewer myocardial infarctions in the ASA group (5.9%) than in the placebo group (13.7%). The difference, however, was not statistically significant (p = 0.10). We were thus unable to demonstrate any favorable influence of ASA 1000 mg daily in patients with reversible ischemic attacks. This study does not, of course, prove that ASA treatment is ineffective in stroke prevention.