ObjectiveTo estimate the effects of prolonged antiplatelet therapy on the primary and secondary incidence of vascular disease.Data sourcesTwenty‐five randomised trials in 29 000 patients with a history of vascular disease (the Antiplatelet Trialists' Collaboration) and two randomised trials in 27 000 individuals without a history of vascular disease (the British doctors' and American physicians' studies).Study selectionThe Antiplatelet Trialists' Collaboration obtained data from all randomised trials of secondary prevention completed before January 1988. The British doctors' and American physicians' studies are the only two completed randomised trials of primary prevention.Data extractionData from the secondary prevention trials were provided by the Antiplatelet Trlalists' Collaboration. Data from the primary prevention trials were extracted from the final published reports of these studies.Data synthesisIn the secondary prevention trials, antiplatelet therapy reduced the rate of vascular disease by about 15% and the incidence of non‐fatal myocardial infarction and stroke by about 30%. In the American physicians' study, but not the British doctors' study, the incidence of non‐fatal myocardial infarction was also reduced. In neither primary prevention trial was there evidence of reduced rates of non‐fatal stroke or vascular death; overall, fatal or disabling strokes were slightly more frequent among those assigned aspirin.ConclusionsFor patients with a history of vascular disease, the benefits of antiplatelet therapy appear to outweigh any risks. Among 100 such patients, antiplatelet therapy for two years would prevent one death and two major non‐fatal events. The balance of benefits and risks for individuals without a history of vascular disease is less clear because there is no firm evidence of a net reduction in either vascular death or disabling non‐fatal vascular events among those treated with aspirin.