BACKGROUND: Significant disparities exist in cardiovascular outcomes based on race/ethnicity and gender. Rates of evidence-based medication use and long-term medication adherence also appear to be lower in racial subgroups and women but have been subject to little attention. Our objective was to evaluate the effect of race/ethnicity and gender on adherence to statin therapy for primary or secondary prevention. METHODS AND RESULTS: Studies were identified through a systematic search of MEDLINE, EMBASE, ClinicalTrials.gov, and the Cochrane Database of Systematic Reviews (through April 1, 2010) and manual examination of references in selected articles. Studies reporting on adherence to statins by men and women or patients of white and nonwhite race were included. Information on study design, adherence measurement, duration, geographic location, sample size, and patient demographics was extracted using a standardized protocol. From 3,022 potentially relevant publications, 53 studies were included. Compared with men, women had a 10% greater odds of nonadherence (odds ratio 1.10, 95% confidence interval [CI], 1.07-1.13). Nonwhite race patients had a 53% greater odds of nonadherence than white race patients (odds ratio 1.53, 95% CI 1.25-1.87). There was significant heterogeneity in the pooled estimate for gender (I(2) 0.95, P value for heterogeneity <.001) and race (I(2) 0.98, P value for heterogeneity <.001). The overall results remained unchanged in those subgroups that had significantly less heterogeneity. CONCLUSIONS: Among patients prescribed statins, women and nonwhite patients are at increased risk for nonadherence. Further research is needed to identify interventions best suited to improve adherence in these populations.