We examined the functional activity of peripheral blood neutrophils and the complement system activation status in patients with rheumatoid arthritis (RA) undergoing infliximab/methotrexate combined therapy. We studied female RA patients under treatment with infliximab (3-5 mg/kg) and methotrexate (15-25 mg/week) who presented inactive (i-RA; n = 34, DAS-28 ≤ 2.6) or at least moderately active disease (a-RA; n = 29, DAS-28 > 3.2), and age-matched healthy women (n = 38). We measured the levels of reactive oxygen species (ROS) generation (chemiluminescence assay) and membrane expression of FcγRIIa/CD32, FcγRIIIb/CD16, CR1/CD35, and CR3/CD11b receptors (ELISA assay) in neutrophils. We also determined the hemolytic activity of the alternative and classical pathways of the complement system (spectrophotometry), serum levels of C5a and Bb (ELISA assay), and serum chemotactic activity (Boyden chamber). Compared with the control group, i-RA and a-RA patients exhibited: (1) increased neutrophil ROS production and membrane expression of FcγRIIa/CD32, FcγRIIIb/CD16, and CR1/CD35, indicating neutrophil activation; and (2) increased serum chemotactic activity and decreased activity of the alternative complement pathway, indicating systemic complement system activation. The levels of C-reactive protein in a-RA patients were augmented, compared with i-RA patients. Although infliximab/methotrexate combined therapy induced disease remission according to the DAS-28 criteria, both i-RA and a-RA patients still exhibited significant levels of systemic activation of neutrophils and the complement system.