OBJECTIVE: To determine whether patients with ankylosing spondylitis (AS) and patients with rheumatoid arthritis (RA) from Japan have antibodies to Klebsiella pneumoniae and Proteus mirabilis and to assess whether such antibodies are activated against peptides sharing sequences with HLA-B27. METHODS: Serum samples from 152 Japanese patients, 52 with AS, 50 with RA, and 50 healthy controls, were tested against 3 bacteria (K. pneumoniae, P. mirabilis, and Escherichia coli) and 3 synthetic peptides (HLA-B27, pullulanase-D, and scrambled pullulanase-D control peptide) by ELISA under coded conditions. Samples were tested for elevations in IgG, IgA, and IgM antibody classes in patients with active AS or RA, in patients with RA with probable disease, and in patients with inactive AS. Disease activity was determined by an elevated serum C-reactive protein (> 10 mg/l) level and elevated erythrocyte sedimentation rate (> 20 mm/h). RESULTS: Patients with active AS showed specific elevations in serum IgA antibody levels against K. pneumoniae compared to patients with RA and controls (p < 0.001). No such elevation was seen in the IgG and IgM antibody classes. Patients with inactive AS showed no elevation in any class of antibody against K. pneumoniae compared to controls or patients with RA. Patients with active or probably active RA showed significant elevations in IgG antibody levels against P. mirabilis compared to AS and controls (p < 0.001). Patients with AS (active or inactive), RA (active or probably active), and controls showed no elevations in any antibody class to E. coli. Both active and inactive AS patients had specific autoantibodies against HLA-B27 peptide compared to patients with RA and controls (active AS: IgG, IgA, IgM, p < 0.001; inactive AS: IgG and IgA, p < 0.001). Patients with active AS had IgG and IgA antibodies against pullulanase-D peptide, which contains a sequence that cross reacts with HLA-B27 compared to controls (p < 0.001). CONCLUSION: These results provide the first evidence of AS and RA patients in Japan having specific elevations of antibody to K. pneumoniae and P. mirabilis, respectively. This suggests that K. pneumoniae in AS and P. mirabilis in RA may play a role in triggering and/or exacerbating these diseases.