Serological tests of 35 patients suffering from inflammatory bowel disease were compared to those of 35 healthy controls. The tests were performed using the indirect immunoperoxidase assay. Ninety-three per cent of 15 patients with Crohn's disease had IgG antibodies against Chlamydia, compared to 26% in the control group. In the 20 patients with ulcerative colitis, 45% had IgG antibodies against Chlamydia, compared to 10% in the control group. High serum titres of IgG antibodies were found in most of the patients with inflammatory bowel disease, mainly with Crohn's disease, while weak reactions appeared in most of the controls in which antibodies were detected. These results suggest a high incidence of Chlamydia infection in the studied patients with inflammatory bowel disease, especially in those with Crohn's disease. The possible association between Chlamydia trachomatis and inflammatory bowel disease is discussed.