Factors underlying spontaneous cardiac rupture were studied in a consecutive series of 30 hearts with ruptured infarcts removed at medicolegal autopsy. Normal and diseased heart muscle and narrowed coronary arteries were examined microscopically. The average age of the 15 women at death was 80 years, and of the 15 men, 73 years. All of the ruptures occurred through a recent transmural left ventricular myocardial infarction which was associated with coronary arteries severely narrowed by atherosclerosis. Nine (33 percent) of the cases showed an occlusive coronary thrombosis. Many of the subjects had no symptoms of recent myocardial ischaemia.