We aimed to determine optimal energy levels for external cardioversion (ECV) in patients with persistent atrial fibrillation (AF) to potentially decrease myocardial damage due to shock and to shorten the procedure time with the object of reducing failed attempts. The study group involved 72 patients with persistent AF (mean time 143±112 days) who had not developed any intracardiac thrombus. All patients had already undergone anticoagulant therapy before ECV for either a long or a short term. They underwent ECV with 100 J, 200 J, 300 J, 360 J and 360 J step-up protocols until they achieved sinus rhythm (SR). Only 2 of 16 cases achieved SR with 100 J (12%). The remaining 70 patients received 200 J of whom only 14 could attain SR (20%). 39 of 59 cases who underwent 360 J achieved SR. 17 failed cases were exposed to 360 J for the second time by changing the electrode position and applying mild pressure to thorax. Five of the patients achieved SR (29%). The overall achievement rate for persistent AF sufferers was determined to be 83%. Application of ECV with 100 J and 200 J failed to provide sufficient SR in chronic AF sufferers (22%). We suggest that starting ECV with 100 J or 200 J will not only increase the number of shocks but also lead to extension of the procedure and, hence, to the likelihood of high myocardial damage. We therefore suggest that starting ECV with 360 J in persistent AF sufferers might be the optimal initial energy.