In 109 patients with chronic diarrhea colonic biopsies were examined for the presence of specific microorganisms. A positive result was obtained in 48% of patients with ulcerative colitis, 50% with Crohn's disease, 21% with non-specific colitis and 36% with non-specific proctitis. Chlamydiae were found most frequently in all groups. Obligate enteropathogenic bacteria were isolated in only three cases of nonspecific colitis. Of the facultatively enteropathogenic organisms Klebsiella and Pseudomonas aeruginosa were isolated in 31% and 24%, respectively, of patients with ulcerative colitis, in 21% and 7% of patients with Crohn's disease, and in 10% and 6% of patients with non-specific colitis. Whereas chlamydial proctitis is a well-known disease, the results of this study point also to a possible pathogenic role of chlamydiae in the proximal colon. Facultatively enteropathogenic organisms causing acute diarrhea might have aetiologic relevance in some cases of chronic non-specific colitis.