Improving patient adherence to medication regimens is an important public health and clinical goal. Numerous studies have shown that non-adherence to medication is associated with increased hospitalization, progression of disease, and higher mortality. There are many practical and behavioral factors that influence non-adherence; one of the top reasons cited is patient forgetfulness. Automated telephonic reminders are among the most common technologies used to help improve patient medication adherence.  OBJECTIVES: To evaluate the effectiveness of an automated telephonic reminder system at improving patient adherence to medications used to treat chronic conditions. METHODS: We conducted a test-control cohort analysis using administrative data from a large chain pharmacy. The cohorts included patients with maintenance prescriptions that were scheduled to be refilled in April 2010. The intervention group had 82,150 patients from Colorado; these patients received refill reminder calls, as needed, over 12 months. The control group included 182,326 patients from Washington State; these patients did not receive reminder calls. Other than location, both groups had similar demographic characteristics. Medication adherence and persistence were calculated and compared between the intervention and control groups in a 12-month follow-up. Medication adherence was evaluated using both continuous and categorical Medication Possession Ratio (MPR) measures. Clinical adherence was defined as MPR≥80%. Medication persistence was measured as days on therapy and percent of patients remaining on therapy. RESULTS: At 12-month follow-up, the MPR for the intervention group was 2.4% higher than the control group (p<0.001), and the intervention group had a 2.3% higher proportion of clinically adherent patients (p<0.001). Persistence for the intervention group was 7.8 days higher than the control group (p<0.001), and 51.3% of patients in the intervention group stayed on therapy compared to 49.6% in the control group (p<0.001).  CONCLUSIONS: Automated telephonic reminders significantly improved patient adherence to medications used to treat chronic conditions.