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OBJECTIVE: To determine whether the need to use doses of infliximab greater than 3 mg/kg every 8 weeks to achieve or maintain clinical response in patients with rheumatoid arthritis (RA) is associated with differences in baseline clinical characteristics or anti-infliximab antibodies. METHODS: Baseline clinical characteristics and anti-infliximab levels were evaluated retrospectively in a cohort of 51 consecutive patients with RA treated with infliximab at a single center. Patients were divided into 2 groups for comparison: Group 1 patients achieved and maintained clinical responses with infliximab 3 mg/kg every 8 weeks; Group 2 patients required higher doses. RESULTS: Thirty-two (63%) patients required infliximab dose escalation (Group 2). There were no statistically significant differences in baseline or clinical characteristics between Group 1 and Group 2 patients. Anti-infliximab antibodies occurred in 47% of Group 2 versus 27% of Group 1 patients, with higher anti-infliximab antibody concentrations in Group 2 patients (mean +/- SD: 18.3 +/- 8.9 g/ml vs 7.5 +/- 4.8 g/ml; p = 0.02). Patients who developed anti-infliximab antibodies were younger and receiving less prednisone at the time of infliximab initiation than patients who did not. CONCLUSION: Finding higher anti-infliximab antibody concentrations in patients who needed dose escalation of infliximab to achieve or maintain clinical responses with lower serum trough levels of infliximab suggests that development of anti-infliximab antibodies may reduce clinical efficacy of infliximab in some patients with RA.