Patients with acute myocardial infarction and evidence of heart failure or left ventricular dysfunction during the acute phase have an excessive mortality risk. Therapy with angiotensin-converting enzyme inhibitors attenuates the detrimental effects of angiotensin II and has been shown to substantially reduce morbidity and mortality in this population. Selective, angiotensin type 1 receptor antagonism with losartan, which inhibits the effects of angiotensin II regardless of its source at the receptor level, may provide more complete blockade of the renin-angiotensin system. The Optimal Therapy in Myocardial Infarction with the Angiotensin II Antagonist Losartan (OPTIMAAL) study is a multicenter, double-blind, randomized, parallel, captopril-controlled trial. The primary hypothesis is that, compared with captopril, losartan will decrease the risk for all-cause mortality by 20% in high-risk patients after acute myocardial infarction. The study population will consist of 5,000 patients, > or = 50 years of age, with heart failure during the acute phase or with a new Q-wave anterior infarction or reinfarction. Patients will be randomized to treatment with either losartan or captopril. All patients will be followed until 937 deaths occur (event-driven). The primary end point is total mortality (all-cause mortality). The secondary and tertiary end points are sudden death (and/or resuscitated cardiac death) and fatal/nonfatal reinfarction. Based on the assumed event rate, treatment effect and a 95% power to detect a 20% reduction in all-cause mortality at the 4.3% significance level (2-sided, adjusted for 2 interim analyses), the trial will enroll at least 5,004 patients and continue until a total number of 937 events has been reached (intention-to-treat analysis).