The aim was to study the serum level of the carcinoma-associated antigen CA 50 as a marker of dysplasia in ulcerative colitis. Seventy-four patients with a mean history of ulcerative colitis of 16 years (SD, 9 years) underwent colonoscopic examination of the entire colon. Dysplasia was diagnosed by light microscopy of multiple biopsy specimens obtained during colonoscopy. Increased serum levels of the antigen CA 50 were found in four patients, of whom two had no signs of dysplasia. Six out of eight patients with moderate dysplasia had serum levels of CA 50 not exceeding a reference level determined as the mean + 2 SD of the results in sera of 500 blood donors. Of 5 patients with ulcerative colitis for more than 30 years, 2 had increased levels of CA 50, whereas only 2 out of 69 with shorter disease duration did (p less than 0.02). Longitudinal studies are required to determine whether measurement of carcinoma-associated antigens will provide clinical information for the treatment of patients with ulcerative colitis.