The population pharmacokinetics of infliximab were characterized in patients with active ankylosing spondylitis (n =274). Serum infliximab concentration data, from a 2-year period, were analyzed using NONMEM. A 2-compartment linear pharmacokinetic model was chosen to describe the pharmacokinetic characteristics of infliximab in serum. Population estimates (typical value ± standard error) were obtained from the final covariate model: clearance (CL: 0.273 ± 0.007 L/day), volume of distribution in the central compartment (V1: 3.06 ± 0.057 L), intercompartment clearance (Q: 1.72 ± 0.48 L/day), and volume of distribution in the peripheral compartment (V2:2.94 ± 0.17 L). Interindividual variability for CL and V1 was 34.1% and 17.5%, respectively. White blood cell count at baseline and the antibody-to-infliximab status were significant covariates to CL; body surface area and sex were significant covariates to V1. The CL for patients with a positive antibody-to-infliximab status was estimated to be 41.9% to 76.7% higher than for the remaining patients. Other covariates (baseline disease activity and the concomitant medication use of pred-nisolone, omeprazole, nonsteroidal anti-inflammatory drugs, or analgesics) did not affect infliximab pharmacokinetics. The development of antibodies to infliximab was associated with accelerated infliximab clearance and may represent a potential underlying mechanism for an inadequate response, or loss of response, to infliximab treatment.