A randomized, double-blind, multicenter trial of azithromycin versus placebo among adults with documented prevalent coronary artery disease. Patients were enrolled over an 18-month period from a total of 28 centers. Following enrollment, an electrocardiogram was obtained, and patients with a prolonged QT interval were excluded. Eligible patients were then randomized to receive either placebo or azithromycin 600 milligrams orally once a week for a year. At the time of enrollment, a blood sample was obtained for C. pneumoniae antibody testing. Patients were contacted at one, three, and six weeks, and at three, six, nine, and twelve months. The patients were followed for a mean of four years for the composite primary outcome of coronary heart disease death, non-fatal myocardial infarction, hospitalization for unstable angina, and requirement for coronary artery bypass grafting or percutaneous revascularization. After the first year of the study, each patient was contacted every six months to determine the occurrence of outcomes. In addition, when available, computerized hospitalization and outpatient data were reviewed. Outcome events were classified using standardized algorithms. The relationship of antibody titer at baseline to the efficacy of azithromycin and outcome was a secondary analysis. The primary analysis was according to the intent-to-treat principle.