One female patient--with slight pure mitral stenosis, mild hypertension and ischemic cardiomyopathy and disabling pulmonary emphysema--developed at 54 years of age permanent atrial fibrillation, had a gratuitous mitral commissurotomy four months later, sustained chronic fibrillation for 13 years, then spontaneously resumed sinus node command at age 67 without any discernible reason. Sinus rhythm was being maintained at follow-up nine months later. Her cardiac status of fair compensation under modest digitoxin and diuretic therapy has neither improved nor worsened with the return of atrial systole. The duration, in this observation, of permanent auricular fibrillation before spontaneous return of sinus rhythm, is one of the longest ever published, exceeded, to the best of my knowledge, only by one case of Lewis and by another one of Reeve and associates. Such an exceptional event points out a fascinating enigma: how can major longstanding atrial dysrhythmias (fibrillation, flutter), whose causes and pathogenesis seem at least partly elucidated, spontaneously disappeer in atria so badly diseased? I think we must humbly confess that no satisfactory explanation is at present available for this disconcerting phenomenon.