BACKGROUND: Biological agents for anti-tumor necrosis factor-α therapy have revolutionized treatments for autoimmune diseases; however, approximately 20% of rheumatology and 40% of gastroenterology patients do not respond to the therapy, or they show reduced drug efficacy because of anti-drug antibody (ADA) formation. OBJECTIVES: To evaluate laboratory tools for individual monitoring of infliximab therapy and the relationship between ADA and infliximab serum levels, ADA and clinical response, and ADA and autoantibodies. METHODS: Our study comprised patients treated with infliximab and affected by selected rheumatology and gastroenterology diseases. Sera were analyzed for infliximab, total-anti-drug antibodies (Total-ADA), and free-anti-drug antibodies (Free-ADA) serum levels and for the detection of specific autoantibodies. RESULTS: We analyzed 73 patients. Total-ADA were detected in 26 rheumatology and 21 gastroenterology patients. Serum infliximab levels were significantly lower in Total-ADA positive patients (P = 0.01 for rheumatology group, P = 0.02 for gastroenterology group). A lack of response was observed in 7 rheumatology and 15 gastroenterology samples. Total-ADA serum levels were statistically significantly higher in patients with treatment failure in both groups (P = 0.01 and P = 0.001, respectively). There was no significant association between the presence of Total-ADA and other autoantibodies. Free-ADA were detected in only 27 rheumatology patients. Results showed a significant correlation with clinical outcome (P = 0.006). CONCLUSIONS: The correlation with clinical response suggests that the presence of ADA could interfere with efficacy of therapy. The tests for monitoring therapy may be an important tool to assist clinicians in early detection and prevention of therapy failure.