The proliferative activity of the colonic crypts of patients who are at high risk of developing colonic tumours has been assessed. Cells from the upper parts of the colonic crypts were collected at colonoscopy, and were stained for DNA by the Feulgen technique. Using a microdensitometer the DNA content of the cells was measured and a proliferative index calculated. Patients with single neoplasms had a proliferative index similar to controls although individual patients with hyperplastic mucosa could be identified. Patients with multiple neoplasms had an elevated proliferative index. These results may form the basis of a screening programme for colonic tumours.