We studied rectal cell proliferation by means of bromodeoxyuridine labelling and ornithine decarboxylase activity assay in 16 patients with colorectal adenoma. In each patient, three rectal biopsy specimens taken from normal-appearing mucosa were incubated with bromodeoxyuridine (BrdU), fixed in ethanol and stained with avidin-biotin peroxidase complex using a monoclonal antibody against BrdU. In addition, two biopsies were homogenized and incubated with [1-14C]-ornithine for ornithine decarboxylase (ODC) assay. A direct, significant correlation was found between BrdU-labelling index and ODC levels in the mucosa (r = 0.6511, P less than 0.01). We conclude that BrdU labelling and ODC activity assay give comparable results in the analysis of cell proliferation rate of rectal mucosa. These methods are useful to investigate rectal cell proliferation pattern of patients with increased risk of colorectal cancer.