A critical review of published literature was performed to assess the impact of medication adherence on morbidity and mortality among patients with or at risk for coronary artery disease and congestive heart failure. Twenty-one original research articles that met our inclusion criteria and related medication adherence to morbidity and mortality are summarized. No clinical trials that specifically tested the impact of a compliance-enhancing intervention on outcome in coronary heart disease were identified. Among 12 studies that compared hospitalization rates and mortality between adherers and nonadherers, 7 showed a significant relationship between medication adherence and outcomes. Three studies showed that adherence to placebo was associated with improved outcomes, suggesting that adherent behavior may be a marker of better prognosis or confers a protective effect on patients with coronary heart disease. Further study is necessary to determine whether adherent behavior can be taught and whether compliance-enhancing strategies improve outcomes in coronary heart disease.