Fifty-three of 4,369 patients with acute myocardial infarction died of myocardial rupture. The incidence of rupture varied directly, among men, with the systolic blood pressure on admission to the coronary care unit (CCU), and the highest systolic pressure while in the CCU. Rupture occurred in 0.3% of the men with systolic pressures on admission to the CCU between 110-129 mm Hg, increasing to 2.0% of men with pressures between 170-189 mm Hg. Similarly, 0.3% of the men with a highest systolic pressure less than 150 mm Hg had a rupture, while 1.6% of those with pressures between 170-189 mm Hg ruptured. Diastolic blood pressure, past history of hypertension, and sustained hypertension after infarction were not related to the occurrence of rupture. Eighteen of the 53 patients who sustained rupture had systolic hypertension (greater than or equal to 150 mm Hg) sometime during the 24 hours before rupture, and 14 had diastolic hypertension (greater than or equal to 95 mm Hg). Hypertension appears to be one of several variables interacting to influence the occurrence of myocardial rupture.