Eleven patients, 10 of whom had ulcerative colitis or proctitis of at least 5 years’ duration, were examined by double contrast enema and by colonoscopy with multiple biopsies of the colonic mucosa. Roentgenology underestimated the extent or severity of the disease as assessed by biopsy in 8 out of 11 patients; for example, in 1 patient with roentgenologic evidence of involvement of the ascending and proximal transverse colon, biopsies showed a total colitis. In 2 patients, colonoscopy detected pseudopolyps and polyps which were not revealed by the double contrast barium enema examination.