We would advocate 3 weeks of anticoagulation prior to, and 4 weeks post-cardioversion (either electrical or chemical) for patients in chronic atrial fibrillation or flutter. In selected cases it seems reasonable to use transoesophageal echocardiography to exclude preformed thrombus and negate the need for 3 weeks of prior anticoagulation. For patients presenting acutely with atrial fibrillation or flutter we suggest anticoagulating with heparin immediately on presentation and for those who do not spontaneously revert to sinus rhythm, using transoesophageal echocardiography to exclude atrial thrombi prior to cardioversion. Oral anticoagulation should be continued for 4 weeks post-procedure. If transoesophageal echocardiography is not readily available an alternative strategy would be to anticoagulate the patient for 3 weeks and thereafter readmit them for elective cardioversion, continuing the anticoagulation for a further 4 weeks after the procedure.