During the past 10 years we experienced 234 patients with atrial fibrillation of whom 94 patients (40.2%) showed a paroxysmal form. Underlying diseases of these 94 patients were hypertension and/or coronary heart disease in 51 (54.3%), rheumatic valvular disease in 3 (3.2%), hyperthyroidism in 2 (2.1%), and miscellaneous diseases in 15 patients (16.0%). Idiopathic atrial fibrillation was found in the remaining 23 patients (24.5%). Frequency of paroxysm was variable. paroxysm occurred only once or twice in 35 patients (37.2%) and more than 21 times in 20 patients (21.3%). Duration of a single paroxysm ranged from less than 24 hours in 49 patients (52.1%) to more than 15 days in 7 patients (7.4%). Atrial fibrillation became permanent in 19 (25.3%) of 75 patients who had been observed for a period of more than 1 year.