BACKGROUND People with nonrheumatic atrial fibrillation who have had a transient ischemic attack or minor ischemic stroke are at risk of recurrent stroke. OBJECTIVE The objective of this review was to assess the effect of antiplatelet therapy for secondary prevention in people with nonrheumatic atrial fibrillation and a previous transient ischaemic attack or ischaemic stroke. METHODS The reviewer searched the Cochrane Stroke Group trials register and contacted trialists. METHODS Randomised trials comparing an antiplatelet agent with placebo or open control in people with nonrheumatic atrial fibrillation and a previous transient ischaemic attack or minor ischaemic stroke. METHODS One reviewer extracted the data. RESULTS One trial was included, in which 300 milligrams of aspirin per day was compared with placebo. This review includes 404 aspirin-treated patients and 378 placebo patients in total. The mean follow-up was 2.3 years. No difference was shown between aspirin and placebo in the annual rate of all vascular events, including vascular death, recurrent stroke (ischaemic or haemorrhagic), myocardial infarction, and systemic embolism. The odds ratio was 0.84, 95% confidence interval 0.63 to 1.14, or 15% of those receiving aspirin versus 19% for those given placebo. Aspirin may prevent 40 vascular events (of all types) per 1000 patients treated for one year. There was a non-significant reduction in the risk of recurrent stroke from 12% to 10% per year (odds ratio 0.89, 95% confidence interval 0.64 to 1.24). The incidence of major bleeding events, requiring hospitalisation, blood transfusion or surgical treatment, was low (0.9% per year for aspirin versus 0.7% for placebo). CONCLUSIONS Aspirin may reduce the risk of vascular events in people with nonrheumatic atrial fibrillation, but the effect shown in the single trial was not statistically significant.