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The literature on patient adherence to treatment includes hundreds of empirical studies. A comprehensive examination of the findings requires the organization and quantification that is possible with meta-analysis.The goals of this research are retrieval, compilation, and averaging of adherence rates in all published empirical studies from 1948 to 1998; assessment of variation according to sample characteristics, time period of publication, measurement method, disease, and regimen; and examination of the effects on adherence of patient demographic characteristics.We calculated a meta-analysis of 569 studies reporting adherence to medical treatment prescribed by a nonpsychiatrist physician, and 164 studies providing correlations between adherence and patients’ age, gender, education, and income/socioeconomic status; group comparison and multiple regression analysis of moderators.The average nonadherence rate is 24.8%. Controlling for intercorrelations among moderator variables, adherence is significantly higher in more recent and smaller studies and in those involving medication regimens and adult samples. The use of physical tests and self-report have respectively significant and borderline negative effects on the level of adherence, and disease severity and use of the medical record have no significant effects. Adherence is highest in HIV disease, arthritis, gastrointestinal disorders, and cancer, and lowest in pulmonary disease, diabetes, and sleep. Demographic effects on adherence are small and moderated by sample, regimen, and measurement variables.This review offers insights into the literature on patient adherence, providing direction for future research. A focus on reliability and validity of adherence measurement and systematic study of substantive and methodologic moderators are recommended for future research on patient adherence.