Improved application of existing therapies, directed by evidence-based guidelines, may offer immediate savings of life and function to patients with cardiovascular disease. We sought to evaluate the evidence that use of performance measures derived from clinical practice guidelines is associated with better clinical outcomes in this context. We conducted a search of the MEDLINE database for published studies evaluating the relationship of evidence-based therapies and outcomes in patients with coronary artery disease or heart failure. Studies examined included single-center, regional, national, and international experiences, and varied considerably in design. Most studies linking guidelines-based care to outcomes focused on patients with coronary artery disease; relatively few addressed patients with heart failure. Few studies, all nonrandomized, examined the use of specific interventions to improve quality of care in the context of standardized care tools. Almost all studies showed a strong and "dose-response" association between adherence to guidelines and performance measures and outcomes. Higher quality of care, as documented by better performance based on measures derived from practice guidelines, is associated with improved outcomes in patients with cardiovascular disease.