This study assessed whether the relative risk of serious infections from infliximab versus etanercept varies by patient characteristics. The study used data from a large US claims database to compare rates of serious infection requiring hospitalization in patients initiating infliximab or etanercept. Cox models adjusting for potential confounders were used to estimate hazard ratios for serious infection associated with each treatment overall and stratified by age, sex, race, BMI, and smoking status. The results suggested the increased risk of serious infection with infliximab compared to etanercept may be modified by age, with a higher risk seen only in patients under 65, but not other characteristics examined. Limitations included potential residual confounding and exposure