OBJECTIVES To determine dosing patterns associated with real-world treatment of rheumatoid arthritis (RA) patients with infliximab or abatacept. METHODS An observational, retrospective (cohort) study of patients new to abatacept and infliximab was conducted using the PharMetrics Patient-Centric Database. All adult patients with at least one claim of RA diagnosis at or prior to initial treatment with infliximab or abatacept from March 2006 to June 2007 were selected. Patients were identified and followed for at least 6 months based on their first infusion claim for infliximab or abatacept with no claims for any other biologic in the prior 6 months. Abatacept and infliximab cohorts were compared with respect to baseline characteristics and occurrence of upward dose adjustments (increase in dose or frequency). RESULTS Forty abatacept and 216 infliximab patients were identified as new to biologic therapy. The two cohorts were generally similar, however 47.5% of initial infusions for abatacept patients were prescribed by a rheumatologist (also, 15% by primary care physician (PCP) and 37.5% unknown), compared to 72.7% for infliximab patients (with 4.6% by PCP and 22.6% unknown). Abatacept patients were less likely to experience upward dose adjustment than infliximab (10% vs. 57.9%, respectively). Multivariable Cox proportional hazards modeling (adjusted for age, gender, Charlson Comorbidity Index, and 1-year pre-index RA-related costs) determined that infliximab patients were more likely to experience upward dose adjustment than abatacept patients (HR=5.5, 95% confidence interval=2.0-14.9, p=0.001). CONCLUSIONS Upward dose adjustment with some biologic therapies is common and may lead to unexpectedly higher treatment costs with additional safety considerations. In this study, upward dose adjustment appears to be less likely in patients started on abatacept than infliximab. Further research should determine if the fixed dosing pattern observed with abatacept continues over time, as health care providers and patients become more familiar with this biologic.