Nonadherence to prescription drugs results in poorer control of chronic health conditions. Because of significant racial/ethnic disparities in the control of many chronic diseases, differences in the rates of and reasons for medication nonadherence should be studied. 1) To determine whether rates of and reasons for medication nonadherence vary by race/ethnicity among seniors; and 2) to evaluate whether any association between race/ethnicity and nonadherence is moderated by prescription coverage and income. Cross-sectional national survey, 2003. Medicare beneficiaries > or = 65 years of age, who reported their race/ethnicity as white, black, or Hispanic, and who reported taking at least 1 medication (n = 14,829). Self-reported nonadherence (caused by cost, self-assessed need, or experiences/side effects) during the last 12 months. Blacks and Hispanics were more likely than whites to report cost-related nonadherence (35.1%, 36.5%, and 26.7%, respectively, p < .001). There were no racial/ethnic differences in nonadherence caused by experiences or self-assessed need. In analyses controlling for age, gender, number of chronic conditions and medications, education, and presence and type of prescription drug coverage, blacks (odds ratio OR 1.38; 95% confidence interval CI 1.08-1.78) and Hispanics (1.35; 1.02-1.78) remained more likely to report cost-related nonadherence compared to whites. When income was added to the model, the relationship between cost-related nonadherence and race/ethnicity was no longer statistically significant (p = .12). Racial/ethnic disparities in medication nonadherence exist among seniors, and are related to cost concerns, and not to differences in experiences or self-assessed need. Considering the importance of medication adherence in controlling chronic diseases, affordability of prescriptions should be explicitly addressed to reduce racial/ethnic disparities.