Five hundred and eighty-five patients with threatened stroke were followed in a randomized clinical trial for an average of 26 months to determine whether aspirin or sulfinpyrazone, singly or in combination, influence the subsequent occurrence of continuing transient ischemic attacks, stroke or death. Eighty-five subjects went on to stroke, and 42 died. Aspirin reduced the risk of continuing ischemic attacks, stroke or death by 19 per cent (P less than 0.05) and also reduced risk for the "harder," more important events of stroke or death by 31 percent (P less than 0.05), but this effect was sex-dependent: among men, the risk reduction for stroke or death was 48 per cent (P less than 0.005), whereas no significant trend was observed among women. For sulfinpyrazone, no risk reduction of ischemic attacks was observed, and the 10 per cent risk reduction of stroke or death was not statistically significant. No overall synergism or antagonism was observed between the two drugs. We conclude that aspirin is an efficacious drug for men with threatened stroke.