The medical records of 254 patients with atrial fibrillation were reviewed to determine the incidence of embolic events in relation to type of cardiovascular disease, duration of atrial fibrillation, and use of anticoagulants. During a total follow-up of 833 patient-years in atrial fibrillation, there were 32 instances of systemic embolism: 21 involved the cerebral circulation and 11 were extracerebral. Thirty of these events occurred during 549 patient-years of follow-up without anticoagulation therapy (5.46 of 100 patient-years), while only two embolic events occurred during 284 patient-years on anticoagulants (0.7 of 100 patient-years). Thus, the incidence of embolism was eight times more frequent during the unanticoagulated period of observation in atrial fibrillation (p less than 0.002). The incidence of embolism during follow-up without anticoagulants was the same regardless of the presence or absence of mitral valve disease and regardless of whether atrial fibrillation was chronic or paroxysmal. The rate of serious hemorrhagic complications on anticoagulants was acceptably low (2.11 of 100 patient-years). We conclude that in this study population anticoagulant therapy reduced the risk of embolic complications of atrial fibrillation. The results also indicate that the use of anticoagulants should not be limited to patients with atrial fibrillation due to mitral valve disease.