604 Patients with atherothrombotic cerebral ischemic events (transient, 16%: or completed, 84%) referrable either to the carotid or to the vertebral-basilar circulation were entered into a double blind randomized clinical trial (AICLA) to determine whether aspirin (A) (1 g/day) or aspirin (1 g/day) + Dipyridamole (225 mg/day) (AD) would produce a significant reduction in the subsequent (3 years) occurrence of fatal and nonfatal cerebral infarction. Randomization produced remarkably comparable treatment groups and this good comparability was maintained throughout the study. Adherence to the protocol and drug compliance were excellent. Side effects, particularly symptoms of peptic ulcer and hemorrhagic events were significantly (p less than 0.03) more frequent in the two treatment groups containing aspirin. With the exception of patients who withdrew from the study, each patient was followed for 3 years. At the end of the study, the number of fatal and nonfatal cerebral infarctions was 31 in the placebo (P) group, 17 in the A group and 18 in the AD group. Taking into account the duration of follow-up for each patient, these figures correspond to cumulative rates of 18% in the P group and 10.5% in each of the 2 active treatment groups. Analysis with the Mantel Method showed: 1)–A difference at the 6% level between the 3 groups and between P and AD; 2)–A difference at the 5% level between P and A; 3)–No difference between (A and AD; 4)–A difference at the 2% level between the P group and the two treated groups taken together (A + AD). Among other diseases occurring during the trial, the only significant difference concerned myocardial infarction, which was less frequent in the 2 treated groups (P less than 0.05). Subgroup analysis failed to show a significant sex difference in the efficacy of aspirin. It is concluded that, in patients comparable to those defined in the protocol, Aspirin (1 g) has a significantly beneficial effect in the secondary prevention of atherothrombotic cerebral infarction.