In this study, we investigated the effect of the antitumor necrosis factor alpha (anti-TNF-alpha) antibody, infliximab, combined with methotrexate (MTX) and MTX alone on the serum levels of interleukin (IL)-23 and IL-17 in rheumatoid arthritis (RA) patients. Infliximab combined with MTX was administered to 26 patients with RA (infliximab group), and MTX alone was given to 20 patients with RA (MTX group). We evaluated clinical and laboratory parameters, including the Disease Activity Scores of 28 joints (DAS28) and serum levels of IL-23 and IL-17 at baseline and at 14 and 30 weeks after the initial treatment with these drugs. Single regression analysis was performed between the levels of serum IL-23 and other clinical and laboratory parameters at baseline before the initial treatment with infliximab or MTX. A significant reduction of DAS28 scores was observed in both the infliximab and the MTX group at 14 and 30 weeks after the initial treatment. A significant decrease in serum levels of IL-23 was observed in the infliximab group but not in the MTX group at 14 and 30 weeks after the initial treatment. Serum IL-17 levels did not show a significant change during the follow-up period. At baseline, before the initial treatment with infliximab or MTX, serum IL-23 levels showed a significant correlation with DAS28 and the number of swollen joints. This study indicated that the reduction of serum IL-23 levels in RA patients was a novel action of infliximab.