Although cost estimates for acute myocardial infarction are necessary for decisions about allocating scarce resources, there is a relative paucity of studies that estimate these costs across the entire spectrum of hospitals in actual clinical practice. This study sought to determine the correlates of in-hospital costs for acute myocardial infarction in patients 65 years of age and older. In the Cooperative Cardiovascular Project pilot, medical records were abstracted for acute myocardial infarction hospitalizations in Connecticut from June 1, 1992, through May 20, 1993. In-hospital costs were calculated by multiplying charges from cost centers by the Medicare ratio of cost-to-charge. Among the 2628 patients in the study sample, the total mean in-hospital cost was $14,772, and the median in-hospital cost was $10,409 (twenty-fifth to seventy-fifth percentile, $6960 to $17,225). The largest proportion of the costs were concentrated in room costs (43% of the total). Although several demographic and clinical characteristics were significantly associated with cost, they accounted for only 7% of the variation. In-hospital procedures and adverse outcomes accounted for 53% of the variation.