We evaluated the influence of age and sex on rectal cell proliferation of 69 hospital controls and 66 patients with colorectal adenomas, by means of incubation of rectal biopsies with tritiated thymidine and autoradiography. In particular, we evaluated the labeling frequency in the upper 40% of rectal crypts (0 h), actually considered as a reliable kinetic marker of colon cancer risk. A direct correlation between 0h and age was found in control subjects (P less than 0.02) but not in adenomas patients (P = NS). Moreover, control subjects over 65 years of age showed a shift of the proliferative compartment similar to that observed in the adenomas group. After adjusting for the age, we did not observe any significant effect of the sex of patients or controls on their cell kinetics parameters. Our results are in agreement with those previously reported on smaller series and with epidemiological studies which indicate a high risk for developing colorectal neoplasia in the elderly subjects.