We investigated the influence of antibodies against infliximab and etanercept on the serum trough levels of these agents and the influence of these antibodies on the effectiveness of treatment in patients with rheumatoid arthritis treated with these agents. Forty patients treated with infliximab for 54 weeks and 40 patients treated with etanercept for 32 weeks were enrolled. They were divided into responder and non-responder groups. Serum trough levels of and antibodies against these agents were measured by enzyme-linked immunosorbent assay or radioimmunoassay. Of the 40 patients treated with infliximab, 14 (35%) had anti-infliximab antibodies. Serum trough levels were significantly lower in the non-responder group (14 patients) than in the responder group (26 patients) 6 weeks after initiation of infliximab (p < 0.05). Conversely, titers of anti-infliximab antibody were significantly higher in the non-responder group than in the responder group between 6 and 38 weeks after initiation of infliximab (p < 0.05). Anti-etanercept antibodies were not detected in any patients on etanercept. Serum trough levels of etanercept were not significantly different between the responder (31 patients) and non-responder groups (9 patients). It seems that the appearance of anti-infliximab antibodies might decrease infliximab serum concentrations and, thereby, reduce the agent's effectiveness. The clinical efficacy of etanercept does not appear to be affected by the serum concentrations if it is administered at standard doses.