BACKGROUND: Given the huge burden of coronary artery disease and the effectiveness of medication therapy, understanding and quantifying known impacts of poor medication adherence for primary and secondary prevention is crucial. We sought to systematically review the literature on this topic area with a focus on quantified cost and clinical outcomes related to adherence. METHODS: We conducted a systematic review of the literature between 1966 and November 2011 using a fixed search strategy, multiple reviewers, and a quality rating scale. We found 2636 articles using this strategy, eventually weaning them down to 25 studies that met our inclusion criteria. Three reviewers independently reviewed the studies and scored them for quality using the Newcastle Ottawa Scoring Scale. RESULTS: We found 5 studies (4 of which focused on statins) that measured the impact of medication adherence on primary prevention of coronary artery disease and 20 articles that focused on the relationship between medication adherence to costs and outcomes related to secondary prevention of coronary artery disease. Most of these latter studies focused on antihypertensive medications and aspirin. All controlled for confounding comorbidities and sociodemographic characteristics, but few controlled for likelihood of adherent patients to have healthier behaviors ("healthy adherer effect"). Three studies found that high adherence significantly improves health outcomes and reduces annual costs for secondary prevention of coronary artery disease (between $294 and $868 per patient, equating to 10.1%-17.8% cost reductions between high- and low-adherence groups). The studies were all of generally of high quality on the Newcastle Ottawa Scale (median score 8 of 9). CONCLUSIONS: Increased medication adherence is associated with improved outcomes and reduced costs, but most studies do not control for a "healthy adherer" effect.