This open, parallel-group study compares quinidine and sotalol treatment for maintenance of sinus rhythm after direct current conversion of patients with chronic atrial fibrillation. The patients from 15 centers in Sweden were randomized to sotalol (98 patients) or quinidine (85 patients) after 2 hours of sinus rhythm after direct current conversion. According to primary efficacy assessment, 52% of the patients in the sotalol group and 48% of the patients in the quinidine group remained in sinus rhythm during the following 6-month treatment period (NS). Furthermore, 34% of the patients treated with sotalol and 22% of the patients treated with quinidine relapsed into atrial fibrillation (NS). Heart rate after relapsing into atrial fibrillation was higher in the patients treated with quinidine (109 beats/min) than in the patients treated with sotalol (78 beats/min, p less than 0.001). Patients treated with sotalol were found to be less symptomatic at the time of relapse compared with relapsing patients in the quinidine group. In terms of safety, more patients were withdrawn from quinidine than from sotalol treatment (26% vs. 11%, p less than 0.05), and sotalol was generally better tolerated than quinidine. Twenty-eight percent of the patients treated with sotalol and 50% of the patients treated with quinidine reported side effects (p less than 0.01). The difference was primarily a result of early (within the first month of treatment) gastrointestinal and skin side effects in the group of patients treated with quinidine.(ABSTRACT TRUNCATED AT 250 WORDS)