The study enrolled 128 participants (RA, 96; healthy controls, 17; osteoarthritis, 11; and IgG4-related disease, 4) between March 2014 and July 2017. Blood samples were collected prior to the commencement of treatment, and serumIgG4 levels were determined using a nephelometric assay (levels ≥135 mg/dL were considered elevated). The levels of serumIgG4 and the ratio of IgG4/total IgG in patients with RA were compared with those in healthy controls, patients with osteoarthritis, and patients with IgG4-related disease. Furthermore, the relationship between serumIgG4 levels and RA disease activity was evaluated.