To investigate whether the reported 17% decline in the national rates of acute ischemic heart disease mortality between 1973 and 1978 was attributable to decreased in-hospital mortality for patients with acute myocardial infarction (MI), we surveyed all 63 acute care hospitals in the Boston, Massachusetts, area. Compared with 1973-1974, more 1978-1979 MI patients were admitted to hospitals in metropolitan Boston, and especially to the five university teaching hospitals. Between 1973-1974 and 1978-1979, hospital admission rates decreased for younger patients and increased for older patients, but overall admission rates were almost identical. In-hospital mortality rates from acute MI did not change significantly in any age group. Because overall MI mortality was declining while in-hospital MI mortality was unchanged, the proportion of acute MI deaths that occurred inside the hospital increased from about 30% to about 40%. Although current hospital care undoubtedly benefits many individual patients, this study suggests that improvements in the in-hospital care of acute MI patients are a major explanation for nationwide mortality trends between 1973 and 1978.