OBJECTIVES: The objective of this research report was to examine the relationship between medication adherence levels and health care cost among patients with rheumatoid arthritis (RA). METHODS: This study used a retrospective case comparison design to examine per member per month (PMPM) medical cost. The commercial population of patients with RA was extracted from two large claims data bases between years 2006 and 2009. The case cohort consisted of compliant patients (MPR ≥ 80%) receiving medication management from the Specialty division of a large pharmacy retail chain. The comparison cohort consisted of non-compliant patients (MPR < 80%) from a national benchmark pharmacy and medical claims data base. Using propensity scores, patients were matched on age, gender, risk score, socio-economic status, standard industrial classification code, comorbid conditions, and pre-medication gap. This process resulted in 512 one-to-one match pairs. RESULTS: Patients with RA who were compliant to their medication regimen had 25% lower PMPM medical cost (in-patient, out-patient, professional, and emergency room cost) than patients who were non-compliant ($637 vs. $855 respectively; P=0.0458). The majority of this cost difference was due to in-patient cost which was 46% lower for compliant patients, followed by professional cost which was 15% lower for compliant patients. A closer look at medical cost by levels of compliance reveals that PMPM medical cost decreased at each level of medication compliance described below. Patients with adherence levels less than 40% had PMPM cost of $1024, those with adherence levels between 40% and 80% had PMPM cost of $838, and patients with adherence levels greater than or equal to 80% had PMPM cost of $637. CONCLUSIONS: Medical cost decreases as adherence to the RA medication regimen increases. Given that the cost of treating RA can be extremely expensive, one approach to addressing this financial issue is to target medication adherence.