OBJECTIVES: The purpose of this study is to compare medication adherence in patients initiated on a combination therapy of Calcium Channel Blockers (CCB) and Statins with those on CCB or Statins mono-therapy. METHODS: Using pharmacy claims data of a California Medicaid population from 1995 to 2002, we computed a 90, 180, 270 and 365-day Medication Possession Ratio (MPR) for the purpose of measuring medication adherence. Four patient cohorts are compared: 1) CCB only (N = 33,367); 2) Statins only (N = 27,536); 3) Combination Therapy: CCB + Statin (initiated on CCB, Statin addition) (N = 14,866); and 4) Combination Therapy: Statin + CCB (initiated on Statin, CCB addition) (N = 5944). Patients with both a CCB and a Statin medication filled on the same day (N = 1265) are included in both Cohorts 3 and 4. Patients must have Medicaid coverage for at least six months prior to initial CCB or Statin medication use to be included in the analysis. Continued eligibility for an adequate length of time is required to compute MPR for given days (e.g., 3 months continued eligibility to compute a 90-day MPR). RESULTS: The average 180-day MPR was 54.5% for Cohort 3 and 52.1% for Cohort 4, which is lower than that of CCB and Statin mono-therapy (56.5% and 59.7%, respectively). However, the average MPR for the combination therapy increased to 64.4% with the addition of a CCB and to 65.2% with the addition of a Statin. In general, average MPRs increased over the years. For the Statins-only cohort, the 180-day MPR increased from 50.8% in 1995 to 64.2% in 2001. CONCLUSIONS: Multiple medications appear to reduce medication adherence. For patients using combination therapy of CCBs and Statins, it may be beneficial to develop a combination drug to improve medication adherence.