OBJECTIVES: Comprehensive Medicaid population based studies of adherence and persistence to tamoxifen and aromatase inhibitors (AIs) including anastrozole, exemestane, and letrozole among women with breast cancer are currently lacking.  The purpose of this study was to estimate the adherence and persistence to tamoxifen and AIs among women with breast cancer enrolled across 38 state Medicaid programs.  Factors predicting adherence and persistence to tamoxifen and AIs were also determined.  METHODS: Study population included women aged 18-64 years who were continuously enrolled for a period of three years (2006-2008) in Medicaid managed care program across 38 states.  Incident users of tamoxifen and AIs in 2007 were identified.  To assess adherence (proportions of days covered) and persistence (gap between prescription refills) to tamoxifen and/or AIs among the final sample of women recipients with breast cancer, prescription drug claims from index date to 12-month post index date were studied.  Logistic regression was used to identify predictors of adherence and persistence, respectively.  RESULTS: Roughly 56% of women with breast cancer were adherent with tamoxifen and/or AIs therapy.  Higher age, white race (versus black), geographic location in Northeast region (versus South, Midwest, and West region), stage II or III/IV (versus stage 0), and higher number of outpatient visits were associated with higher adherence in the regression model.  More than three-fourths (~77%) of women with breast cancer were persistent with tamoxifen and/or AIs therapy.  Regression analysis revealed that women with breast cancer residing in Northeast region (versus South and West region) and with stage I or III/IV cancer (versus stage 0) were more likely to be persistent with therapy. CONCLUSIONS:   Adherence and persistent to tamoxifen and AIs was less than optimal among women with breast cancer enrolled in Medicaid programs.  Policy makers should consider implementing interventions aimed at increasing treatment adherence and persistence among this population.