Although cardiogenic shock in acute myocardial infarction is usually associated with a critical loss of myocardium, this may not be the case in papillary muscle rupture. During the past 21 years, 13 patients came to autopsy (11 died in cardiogenic shock) with a papillary muscle rupture complicating myocardial infarction. Rupture occurred from 2 to 7 (mean, 4) days after the infarct, and survival after rupture was usually brief (median, 3 days). The infarct involved between 1% and 50% (mean, 19%) of the left ventricle, and in 10 it was less than 25%. In all instances myocardium around the mitral annulus was not infarcted. Because papillary muscle rupture occurred mostly with first infarcts (eight), involved relatively small areas of necrosis, and spared the myocardium surrounding the annulus, early mitral valve replacement should make this cause of fatal acute myocardial infarction one of the most treatable forms of cardiogenic shock.