This study was carried out to characterize the antibody class response by ELISA to seven Klebsiella pneumoniae serotypes (K2, K3, K17, K21, K26, K36, K50) in five different groups, 40 HLA-B27-positive ankylosing spondylitis (AS) patients, 46 patients with Crohn's disease (CD), 38 patients with ulcerative colitis (UC), 50 patients with active anti-endomysial antibody-positive coeliac disease and 40 healthy controls, using whole bacteria and capsular polysaccharide. IgG antibody levels were significantly elevated in AS patients to K17, K36, K50; IgA to K2, K3, K21, K26, K36 and K50; and IgM to serotype K21 when compared to normal controls. Furthermore, IgG antibody levels were significantly elevated in CD patients to K2, K17, K21, K26, K36 and K50; IgA to K2, K3, K21, K26, K36 and K50; and IgM to K2, K3, K17, K21 and K50. Increased IgG antibody levels in the UC group were limited only to K17, K36 and K50. No antibody class was increased to any of the K. pneumoniae serotypes in the coeliac disease group. The immune responses in AS patients also involve Klebsiella bacteria having capsular serotypes other than K26, K36 and K50. The similarity in the immune responses between CD and AS groups suggests that many AS patients may have occult bowel inflammation.