The occurrence of myocardial rupture was evaluated in an unselected population of 1,737 patients with acute myocardial infarction (AMI). Patients with cardiac rupture after AMI were compared with age- and sex-matched control patients with fatal AMI not related to rupture and with AMI survivors discharged home. Rupture was found in 40 patients (15.7 percent of hospital deaths), or 2.3 percent of all cases of AMI. At the highest risk for rupture were women aged 60 to 69, although the age distribution did not differ significantly from that of patients dying of other causes. More patients with myocardial rupture had hypertension during hospitalization, persistent pain, and inferior wall myocardial infarction when compared with controls. The majority (95 percent) of cardiac ruptures occurred within the first six days, 40 percent within the first 24 hours after the onset of symptoms. Approximately 20 percent of ruptures were diagnosed as subacute; in only two was surgical intervention attempted unsuccessfully. The high-risk group of patients should be carefully monitored within the first six days after the onset of symptoms of AMI in an effort to prevent myocardial rupture.