The benefits of long term anticoagulant treatment of patients with non-rheumatic atrial fibrillation and cerebral infarction were studied by comparing two series of patients with stroke from centres with different policies on anticoagulant treatment. The long term prognosis of 50 patients from the Oxfordshire community stroke project, who did not receive anticoagulants, was compared with that of 70 similar patients from Maastricht, who were treated with anticoagulants. After a mean follow up of 27 months there was no significant difference in either the rate of survival or the rate of recurrent stroke between the two groups. These data suggest that any benefit of anticoagulation is modest. A large randomised trial is planned to establish whether long term anticoagulant treatment is of value and, if so, to what extent.