To quantitate embolic risk we studied a retrospective series of 61 patients with strictly defined bacterial endocarditis. Eighteen patients had neurological complications attributable to cerebral embolism. Seventeen embolic episodes occurred prior to antibiotic treatment and 8 episodes after its commencement. The rate of embolism per patient-week during a 20-week observation period showed a highly significant almost 4-fold reduction to a low level soon after antibiotics were started. Patients already on anticoagulation for prosthetic valves had the same embolic risk as those not so treated. The evidence suggests that anticoagulation at the time of diagnosis of bacterial endocarditis is not warranted.