Cell proliferation kinetics, using autoradiography, was investigated in 4 specimens of normal colonic mucosa, 11 cases of ulcerative colitis, 15 adenomas, and 17 advanced cancers of the large intestine. In the normal mucosa, the labeled cells were located only in the lower three fifths of the crypts, the so-called proliferation zone, and their labeling index was 11.7 +/- 4.0%. In the ulcerative colitis, the proliferation zones were remarkably extended toward the upper part of the crypts, and the mean labeling index was 20.5 +/- 9.4%. In the adenomatous polyps, labeling was seen in any part of the crypts, and abnormal proliferation patterns such as denseness, stratification, expansion, and budding of the labeled cells tended to increase in accordance with the degree of the histologic atypism. Their mean labeling index was 22.2 +/- 8.6%. However each labeling index of the polyps increased proportionally to their histologic atypism, from 15.2 +/- 8.1% in grade II to 30.1 +/- 9.9% in grade V. The mean labeling index of the cancers was 23.3 +/- 9.6%, and there was little difference in the labeling patterns of the malignant polyps and cancers. The proliferation patterns of the polyp and cancer were qualitatively different from those of the normal mucosa and the ulcerative colitis.