The expression of gastrointestinal cancer antigen, CA 19-9, and of carcinoma-associated antigen, CA-50, was studied in formalin-fixed and paraffin-embedded tissue from 18 patients with ulcerative colitis, 29 with Crohn's disease in the colon, four with diverticular disease, and eight with sigmoid volvulus. None of the patients with inflammatory bowel disease showed strong dysplasia or had manifest carcinoma. Both antigens were expressed frequently in patients with inflammatory bowel disease. Of the 18 patients with ulcerative colitis, 17 were positive for both CA 19-9 and CA-50, and of the 29 with Crohn's colitis, 21 were positive for CA 19-9 and 22 for CA-50. No distinct differences in antigenic expression were found between Crohn's disease and ulcerative colitis. CA-50 was expressed in normal colonic mucosa from 10 of 12 patients with sigmoid volvulus or diverticular disease, and such mucosa was positive for CA 19-9 in three of the four patients with diverticular disease and in two of the eight patients with sigmoid volvulus. It is concluded that immunodetection of CA-50 or CA 19-9 is of limited value in the differential diagnosis of inflammatory bowel disease. The usefulness of these antigens as markers for precancerous changes in inflammatory bowel disease is also doubtful, since the expression is also frequent in cases of inflammatory lesions, with no obviously increased risk of malignancy.