We present the cases of four patients with reversible left ventricular dysfunction associated with severe congestive heart failure presumably induced by rapid atrial fibrillation. The mean heart rate was 159 beats/min and the mean left ventricular end-diastolic dimension was 58.5 mm with diffusely impaired left ventricular motion. None of the patients had a history of preceding upper respiratory infection before the acute episode and no signs of inflammation at onset, and all patients were New York Heart Association Class I or II before the acute episode. Left ventriculography, done about 1 month when congestive heart failure and ventricular rate were controlled with digitalis and diuretics, still showed diffusely decreased left ventricular motion; the mean end-diastolic volume was 165 ml and the mean ejection fraction was 30%. Coronary angiography was normal in three patients and one showed moderate left anterior descending artery stenosis. Right ventricular biopsy, done in three patients showed no evidence of myocarditis. Left ventricular wall motion normalized in 5-36 months on follow-up echocardiography. These findings suggest that persistent rapid atrial fibrillation can cause reversible left ventricular dysfunction which can take a considerable period of time to normalize.