The authors evaluated cell kinetics of apparently normal rectal mucosa in the following subjects: 25 with single small adenoma (smaller than 10 mm) of the large bowel, 12 with multiple small adenomas, 28 with a single large adenoma (larger than 10 mm), 22 bearing multiple adenomas among which at least one was larger than 10 mm, 32 with cancer of the large intestine, and 32 controls without colorectal diseases. The study was performed by means of incubation of biopsy specimens with tritiated thymidine and autoradiography. The labeling index was similar in all of the groups. However, patients with one or more large adenomas showed a shift of the proliferative compartment toward the top of the crypts similar to that observed in patients with cancer. This abnormal proliferative pattern was not noticed in patients with one or more small adenomas and was not related to the number of adenomas of each subject. The presence of defects of cell growth in the normal rectal mucosa of patients with large adenomas may indicate that subjects with this abnormality are at high risk of adenoma growth and progression to cancer.