Context: Recent hospital reductions in registered nurses (RNs) for hospital care raise concerns about patient outcomes. Objective: Assess the association of nurse staffing with in-hospital mortality for patients with acute myocardial infarction (AMI). Design, Setting, and Patients. Medical record review data from the 1994-1995 Cooperative Cardiovascular Project were linked with American Hospital Association data for 118,940 fee-for-service Medicare patients hospitalized with AMI. Staffing levels were represented as nurse to patient ratios categorized into quartiles for RNs and for licensed practical nurses (LPNs). Main Outcome Measures. In-hospital mortality. Results: From highest to lowest quartile of RN staffing, in-hospital mortality was 17.8%, 17.4%, 18.5%, and 20.1%, respectively (P < 0.001 for trend). However, from highest to lowest quartile of LPN staffing, mortality was 20.1%, 18.7%, 17.9%, and 17.2%, respectively P < 0.001). After adjustment for patient demographic and clinical characteristics, treatment, and for hospital volume, technology index, and teaching and urban status, patients treated in environments with higher RN staffing were less likely to die in-hospital; odds ratios (95% confidence intervals) of quartiles 4, 3, and 2 versus quartile 1 were 0.91 (0.86-0.97), 0.94 (0.88-1.00), and 0.96 (0.90-1.02), respectively. Conversely, after adjustment, patients treated in environments with higher LPN staffing were more likely to die in-hospital; odds ratios (95% confidence intervals) of quartiles 4, 3, and 2 versus quartile 1 were 1.07(1.00-1.15), 1.02(0.96-1.09), and 1.00 (0.94-1.07), respectively. Conclusions: Even after extensive adjustment, higher RN staffing levels were associated with lower mortality. Our findings suggest an important effect of nurse staffing on in-hospital mortality. Copyright © 2003 by Lippincott Williams & Wilkins.