Left atrial thrombosis is a serious complication of atrial fibrillation because of its embolic potential, especially for the cerebral circulation. These thrombi are usually treated by oral anticoagulation. The authors studied the efficacy and tolerance of a low molecular weight heparin. Enoxaparin, in the treatment of this condition. This was a prospective study carried out over a 1 year period. Patient recruitment came from the transoesophageal echocardiography laboratory: of 15 thrombi detected, 6 were treated by ambulatory Enoxaparin therapy. Five of the 6 patients had no signs of left atrial thrombosis after 3 weeks of Enoxaparin therapy. The left atrial thrombus of the 6th patient remained hyperechogenic and had decreased in length from 27 to 24 mm. No cases of bleeding, haematoma, embolism or thrombopenia were observed with this treatment. This preliminary study shows that low molecular weight heparin may be used as an alternative to classical oral anticoagulation for the treatment of left atrial thrombosis.