Transesophageal echocardiography (TEE) guided early cardioversion (CV) in conjunction with short-term anticoagulation has been shown to be safe, and an alternative to prolonged conventional anticoagulation therapy. Recently, low molecular weight heparins (LMWHs) have been used successfully as an alternative to standard heparin therapy obviating the need for hospitalization and APTT monitoring. The aim of this study was to determine the feasibility and safety of TEE guided early cardioversion in conjunction with short-term LMWH use in patients with nonvalvular atrial fibrillation (NVAF). The study group consisted of 172 consecutive patients with NVAF. Before TEE, 90 patients received LMWH (Dalteparin 2 × 5,000U) and 82 patients received standard heparin (UFH) (5,000U bolus followed by infusion to raise APTT to 1.5 times control). TEE was performed and the left atrium and left atrial appendage were examined thoroughly for the presence of thrombus. One patient from each group was excluded due to detection of a left atrial thrombus by TEE. Immediately after TEE, CV was attempted and warfarin was initiated. All patients received warfarin for one month after CV. In the LMWH group, 89 of 90 patients (98.9%) were successfully cardioverted. CV was successful in 97.5% of the patients in the UFH group. None of the patients experienced thromboembolic events during the four weeks after CV. TEE guided early CV in conjunction with short-term LMWH treatment is as safe as UFH for the prevention of thromboembolic events after CV. Copyright © 2003 by the Japanese Heart Journal.