The records of 17 patients (10 men and 7 women) with the diagnosis of acute papillary muscle rupture secondary to acute myocardial infarction (AMI) were reviewed to determine the clinical course and pathologic features of this entity. Eight patients underwent operation after papillary muscle rupture had been diagnosed, and 9 had the diagnosis confirmed at autopsy without a prior surgical procedure. The ages were 44 to 80 years (mean 64). The site of AMI was inferolateral in 15 and anterior in 2. The recorded onset of mitral regurgitation ranged from less than 24 hours to 28 days after AMI (mean 6 days). Of the 11 patients presenting with pulmonary congestion alone, 6 remained stable and had subsequent mitral valve replacement and coronary artery bypass graft operation; however, 5 patients' condition initially stabilized with medical therapy and then suddenly deteriorated after a variable period (1 to 60 days), followed by death. Of the 6 patients who presented with systemic hypotension and pulmonary congestion, 4 were treated medically and died; 1 of the 2 who had surgical treatment survived. The extent of the AMI at autopsy was small and was limited to the subendocardium in half of the patients. Significant coronary artery disease was limited to a single vessel in 7 of 14 patients. The unpredictable and rapid clinical deterioration and the limited extent of coronary atherosclerotic disease and infarct size suggest that early surgical repair should be undertaken in patients with papillary muscle rupture after AMI.