Cumulative evidence from past studies supports but does not prove a causal association between Chlamydia pneumoniae infection and development or progression of atherosclerotic cardiovascular disease. Evidence supporting an association raises the question of whether a causal effect of Chlamydia pneumonia infection, if present, may be ameliorated by antibiotic treatment, resulting in decreased CHD outcomes. The ACES is a randomized, double-blind, placebo-controlled study to determine whether treatment with azithromycin decreases the rate of coronary heart disease events among patients with stable documented coronary artery disease.