The aim of this study is to determine whether treatment increases the levels of anti-Proteus antibodies (APA) in patients with rheumatoid arthritis (RA). The blood samples of 32 patients suffering from RA who were recruited in our previous study and continued to participate in our follow-up study were collected after 1 year. Their first and follow-up samples were analysed for the presence of IgG isotype and total immunoglobulins (IgG+IgA+IgM) against Proteus mirabalis (PM) using enzyme-linked immunosorbent assay with two kinds of antigen preparations: whole bacteria and sodium dodecyl sulphate (SDS) lysed bacterial extract. All patients were treated with methotrexate and hydroxychloroquine with adequate dose of non-steroidal anti-inflammatory drug. After 1 year, 11 patients were in clinical remission [erythrocyte sedimentation rate (ESR) less than 30 mm/h and C-reactive protein (CRP) less than or equal to 10 mg/l], while the rest of the 21 were in the state of active disease. Correlation and Student's t test were used for statistical analysis. APA titres were significantly elevated in patients after 1 year of therapy. However, the rise was not different between patients who were in clinical remission and those in the state of active disease. APA titre increases in the treatment of RA, and the probable mechanisms are discussed.