In the Canadian Cooperative Study of aspirin and sulfinpyrazone in patients with threatened stroke, there was no demonstrable benefit of sulfinpyrazone but there was a significant overall risk reduction in stroke or death of 31% with aspirin (p less than 0.05). This benefit of aspirin was restricted to males, in whom the risk reduction in stroke or death was 48% (p less than 0.005). As with a large-scale clinical trials, questions have been raised about the methodology of the study, including the type of patient included, the choice of outcome measures, the factorial design and the analysis of subgroups. In this report, the design and principal results of the study are summarized and the above methodologic concerns are discussed.