A total of 1,975 plasma diltiazem concentrations were obtained from 1,067 patients enrolled in a multicenter secondary intervention study of diltiazem after acute myocardial infarction. To evaluate the determinants and significance of diltiazem concentrations in this patient population, we related drug concentrations to a variety of clinical variables recorded on the case history forms. Multiple linear regression analysis showed that (1) time from the last drug dose, (2) drug dose taken, (3) patient height (an index of lean body weight), and (4) patient age were important determinants of plasma concentration. For an equivalent dose, plasma diltiazem concentrations in a 75-year-old patient were about double those of a 25-year-old patient. Total weight and drug dose prescribed did not significantly affect plasma concentrations. Whereas drug concentrations were higher (p = 0.01) among patients with left-sided heart failure, they were not altered by renal dysfunction, hepatic disease or β blockers. Diltiazem concentrations were a significant determinant of diastolic arterial pressure (p < 10-9), but neither systolic pressure nor heart rate were significantly related to diltiazem concentration. The overall incidence of adverse experiences was not related to drug concentrations, but the occurrence of second- and third-degree atrioventricular block in the coronary care unit and the need for a temporary pacemaker were substantially higher among patients with a drug concentration >150 ng/ml (7.4 and 1.9%, respectively) than among patients with lower concentrations (2.6% for atrioventricular block, 0.3% for pacemaker; p = 0.02 for each). The risk of atrioventricular block was particularly increased by high diltiazem concentrations in the face of acute inferior infarction. These results suggest that diltiazem's pharmacologic and clinical effects in a large population are concentration-related, and that the consideration of patient size, age, and left ventricular function in selecting a diltiazem dose may allow for effective drug therapy with a reduced likelihood of adverse effects.