Rectal biopsy samples from 22 healthy control subjects, 54 patients with ulcerative colitis, and 34 with Crohn's disease with involvement of the colon or rectum were investigated in a stereomicroscopic study. Samples were stained as whole mounts with Alcian Green before the stereomicroscopic examination. In ulcerative colitis, biopsies almost constantly showed changes in the mucosal surface pattern, including irregular, enlarged crypt openings filled with exudate, granulated and bulging surface, and patchy localization of the goblet cells. There was a close correlation between the stereomicroscopic findings and the clinical disease activity, the sigmoidoscopic findings, and the histologic activity. Apart from the stereomicroscopic observation of small superficial erosions in one fourth of the biopsies, no changes of diagnostic value were observed in Crohn's disease.