BACKGROUND: Previous studies suggest that patients hospitalized with acute myocardial infarction (MI) in December have poor outcomes, and some studies have hypothesized that the cause may be the infrequent use of evidence-based therapies during the December holiday season. OBJECTIVE: To compare the care and outcomes of patients with acute MI hospitalized in December and patients hospitalized during other months. DESIGN: Retrospective analysis of data from the Cooperative Cardiovascular Project. SETTING: Nonfederal, acute care hospitals in the United States. PATIENTS: 127 959 Medicare beneficiaries hospitalized between January 1994 and February 1996 with confirmed acute MI. MEASUREMENTS: Use of aspirin, beta-blockers, and reperfusion therapy (thrombolytic therapy or percutaneous coronary intervention), and 30-day mortality. RESULTS: When the authors controlled for patient, hospital, and physician characteristics, the use of evidence-based therapies was not significantly lower but 30-day mortality was higher (21.7% vs. 20.1%; adjusted odds ratio, 1.07 [95% CI, 1.02 to 1.12]) among patients hospitalized in December. LIMITATIONS: This was a nonrandomized, observational study. Unmeasured characteristics may have contributed to outcome differences. CONCLUSIONS: Thirty-day mortality rates were higher for Medicare patients hospitalized with acute MI in December than in other months, although the use of evidence-based therapies was not significantly lower.