Sigmoidal and rectal biopsies were taken from 10 patients at colonoscopy. Colonic crypt cell proliferation was measured using a monoclonal antibody to bromodeoxyuridine by an immunohistochemical technique. Labelling index and crypt cell proliferation profiles were analysed and compared between the two sites. No statistically significant difference was found in proliferation parameters. Assessment of colonic proliferation is a useful indicator for colorectal cancer risk and for determining the effects of ongoing drug treatment. Rectal biopsies are easier to take than sigmoidal biopsies. The results of this study suggest that there is no difference in cell proliferation at the two sites. Rectal biopsies are more practical for clinical studies.