The pooled results from five placebo-controlled postinfarction studies with metoprolol have shown a significant reduction in total mortality. All five studies used immediate-release metoprolol twice daily. An extended-release formulation of metoprolol for once-daily use has since been developed. The aim of the present study was to compare the two different forms of metoprolol with regard to the risk reduction of death for 5 years postinfarction and to analyze whether treatment with the beta-blocker metoprolol is associated with a reduced mortality after the introduction of modern therapies such as thrombolysis, aspirin, and ACE inhibitors. All patients discharged after an acute myocardial infarction (AMI) from Sahlgrenska University Hospital (SU) during 1986-1987 (n = 740, Period I) and during 1990-1991 (n = 1446, Period II) from both SU and Ostra Hospital, Göteborg, Sweden, were included in the study. During Period I, 56% were prescribed immediate-release metoprolol compared with 61% prescribed extended-release metoprolol during Period II. Immediate-release metoprolol was not available for outpatient use during Period II. In a multivariate analysis, all variables significantly associated with either increased or decreased postinfarction mortality during Periods I and II (univariate analysis of patient characteristics, medical history, complications during the AMI medication at discharge) studied were with Cox's proportional hazards model. Treatment with immediate-release metoprolol was significantly associated with reduced mortality over 5 years during Period I (relative risk reduction for total mortality, -34%, P = 0.003; 95% CI for RR, 0.51-0.87), and treatment with extended-release metoprolol was significantly associated with reduced mortality during Period II (-34%, P < 0.0001; 95% CI for RR, 0.53-0.82). Thrombolysis and the use of aspirin and ACE inhibitors were more frequently used during Period II. The results showed that postinfarction treatment with extended-release metoprolol given once daily was associated with a similar risk reduction of death over 5 years as immediate-release metoprolol given twice daily. The data, furthermore, indicate that the beta-blocker metoprolol is associated with a reduced risk of death after the introduction of modern therapy such as thrombolysis, aspirin, and ACE inhibitors.