Certain clinical and cardiac morphologic findings are described in 22 patients, aged 45 to 80 years (mean 64) (15 men [68%]), in whom rupture of a papillary muscle occurred during acute myocardial infarction. In most, the acute infarction associated with papillary muscle rupture was a first coronary event (only 18% had a myocardial scar consistent with prior infarction and 29% had angina pectoris). The posteromedial papillary muscle, presumably because of its more tenuous blood supply, ruptured almost three times more frequently than the anterolateral one (73 and 27%, respectively). Quantitative examination of the amounts of narrowing by atherosclerotic plaque in each of the four major epicardial coronary arteries (right, left main, left anterior descending and left circumflex) disclosed less narrowing in the patients with rupture than in the patients with fatal acute myocardial infarction unassociated with rupture. Of the 519 five mm sections of coronary artery examined (11 patients), only 68 sections (13%) were narrowed greater than 75% in cross-sectional area compared with 34% of 1,403 sections from 27 patients with fatal myocardial infarction without rupture.