OBJECTIVES: Several studies have demonstrated that higher patient out-of pocket cost may result in lower medication adherence. The purpose of this study was to measure Medication Possession Ratio (MPR) among patients newly-prescribed a brand or generic statin medication in a managed Medicaid plan and a commercial health plan. METHODS: We conducted a retrospective analysis using pharmacy claims data to identify patients who were new to statin treatment (no pharmacy claim for a lipid-lowering medication in the previous 12 months). Patients were categorized based on their index medication. We used a Robust linear regression model to determine predictors of adherence. RESULTS: A total of 738 commercial patients and 2175 Medicaid patients were included. Sixty percent of Medicaid patients and 49% of commercial patients initiated therapy with a generic medication. Average patient out-of-pocket cost for commercial plan patients was $9/month for generic and $15/month for brand. Medicaid patients had no copayment for generic or brand medications. In the commercial plan, there was no significant difference in MPR between patients who initiated therapy with generic or brand statins (MPR 0.75 vs. 0.73, respectively). In the Medicaid plan, MPR was significantly higher among patients who started on generic medications (0.69 vs. 0.63). In robust linear regression, MPR was significantly related to age, number of comorbidities and generic use. After we adjusted MPR for age and comorbidities, MPR remained significantly higher in the Medicaid generic group. CONCLUSIONS: Medicaid patients prescribed a generic statin as initial therapy were more adherent than those prescribed a brand, despite having no copayment for generic or brand medications. This difference was not present among commercial plan patients who had a higher cost share for brand medications.  This suggests that additional research is needed to identify non-financial barriers to adherence.