Recent studies have proposed that the exclusion of an atrial thrombus by transesophageal echocardiography (TEE) would allow for the safe cardioversion from atrial fibrillation or flutter without the need of prophylactic anticoagulation. Because all of the TEE trials have been small and descriptive and have lacked randomized, conventionally treated control groups, the pooled risk of embolic events from TEE trials was compared with that of a control group pooled from the literature on cardioversion both with and without conventional anticoagulation. Studies were identified from a MEDLINE search, references in review articles, and recent cardiology abstracts and were included if there were > 10 patients and if atrial fibrillation or flutter was of > 48 hours' duration. Where > 1 study had been published by the same group only the largest study was used. Studies were not selected by cause of arrhythmia, by predisposing risk factors for atrial fibrillation and flutter, or by method of cardioversion. The only patients excluded from TEE reports were those with atrial thrombi diagnosed on the precardioversion TEE or those documented to have adequate standard precardioversion anticoagulation. Seven TEE and 18 control studies met the inclusion criteria. More patients in the control studies had rheumatic valvular disease. Embolic events were significantly more frequent in the TEE group than in the anticoagulated control group (1.34% vs 0.33%, respectively; p = 0.04), whereas there was no significant difference between the TEE group and the nonanticoagulated control group (2.00%; p = 0.26). Thus the use of TEE screening to exclude patients with atrial thrombi before cardioversion does not identify patients who can safely undergo this procedure without anticoagulation.