In a cooperative study of 3,099 life insurance applicants with atrial fibrillation (AF), 71 deaths over an average duration of 3.3 years were observed and compared with deaths expected and derived from standard insurance mortality tables. A normal mortality was observed in cases of paroxysmal AF with no other identified cardiovascular impairment. Paroxysmal AF with associated mitral stenosis or coronary disease was found to have a significantly increased mortality. Chronic AF with or without other impairments entails a much higher risk than paroxysmal AF, those cases associated with mitral stenosis showing very high excess mortality.