Both trials were randomized and double-blind. Prior to randomization, all patients received enalapril, and placebo for two weeks to enable early detection of non-compliant patients, those unable to tolerate the drug, and those with early side-effects. After completion of the pre-randomization period, participants were allocated to enalapril or placebo using a permuted block randomization within each of the clinical centers. Follow-up visits were scheduled for two and six weeks after randomization and then at four, eight, and twelve months. Thereafter, clinic visits were scheduled at four-month intervals. Patients were followed for a minimum of two years and a maximum of five years. Primary outcome for each trial was all-cause mortality. Secondary endpoints included cardiovascular mortality, sudden death due to worsening congestive heart failure, hospitalization for congestive heart failure, myocardial infarction, stroke, need for cardiac transplantation, and quality of life. Onset of congestive heart failure was an additional outcome in the Prevention Trial. Seven substudies were conducted among subsets of participants and included: diastolic function; echocardiography; exercise; neurohumoral; quality of life; radionuclide; sudden death. Patient recruitment began in July 1986. Recruitment in the Treatment Trial ended six months ahead of schedule in February 1989, and follow-up ended in February 1991. The Prevention Trial ended recruitment in April 1990 and completed follow-up in July 1991. Data analysis continued through November 1994.