Rhythm conversion in patients with post-thyrotoxic atrial fibrillation (AF) has been performed with disopyramide in order to evaluate the conversion rate and to test its effect on the maintenance of sinus rhythm after cardioversion. The duration of AF ranged from 9 to 122 months (mean 31.8 months). Of 81 patients, 12 (15%) with relatively short duration AF were converted to sinus rhythm with disopyramide. The remaining 69 patients required DC cardioversion, which restored sinus rhythm in 58 patients. The 58 DC-converted patients were divided into two groups: a disopyramide group (D group) and a non-disopyramide group (non-D group). The D group received disopyramide 300 mg per day for 3 months after DC cardioversion and the non-D group did not receive anti-arrhythmic drugs. During the early observation period, only one patient relapsed in the D group into AF, but 5 successive patients in the non-D group reverted to AF, forcing discontinuation of the non-D protocol. A second DC cardioversion performed on 3 of those 5 patients was followed by maintenance therapy with disopyramide 300 mg per day, and they remained in sinus rhythm. With the inclusion of those three subjects, sinus rhythm was still present in 44 of the total of 58 patients converted by DC (76%) at the time of follow-up (64 months). Thus, disopyramide was effective in rhythm conversion and it was essential for the maintenance of sinus rhythm after cardioversion in patients with post-thyrotoxic AF.