- Background: Variation in case mix of patients can significantly influence outcome. In this study, the management and outcomes of patients with acute myocardial infarction who were admitted either to coronary care units or to internal medicine wards were examined. Methods: A nationwide prospective study was performed during a 2-month period in all 26 coronary care units and in 82 of 96 internal medicine wards in Israel. All patients with a discharge diagnosis of acute myocardial infarction were included. Comorbidity was coded using the Index of Coexistent Diseases. Results: A total of 1648 consecutive patients with acute myocardial infarction were identified. One thousand and eighty eight (66%) were admitted to coronary care units and 560 (34%) to internal medicine wards. The 30-day mortality for the coronary care unit group was 9.2% compared to 15.5% for patients in the internal medicine ward group. Using logistic regression, independent factors determining 30-day mortality were (odds ratio and 95% confidence interval): age (1.06 per year, 1.03–1.08), Killip score (2.09, 1.64–2.67), Q wave acute myocardial infarction (2.12, 1.31–3.43), and Index of Coexistent Diseases score (1.42, 1.12–1.80). After controlling for age, infarct type and severity, and coexisting medical conditions, no excess mortality was detected among patients admitted to internal medicine wards. Conclusions: Variance in the case mix has a great influence on the interpretation of mortality in studies of acute myocardial infarction.