Immunohistochemical detection of abnormal cell proliferation in colonic mucosa of subjects with polyps. Previous studies have shown the presence of increased proliferation in the large bowel epithelium of those at high risk of developing colon cancer. An in vitro technique for labelling large bowel mucosa with the thymidine analogue bromodeoxyuridine (Brdu) was therefore developed and its ability to distinguish differences in mucosal proliferation between subjects with colorectal adenomas and normal controls was assessed. Sigmoid biopsy specimens from 15 subjects with polyps and 15 age and sex matched controls were labelled and the incorporated Brdu visualised with an immunohistochemical technique. Mean labelling index (LI) was significantly higher in those with polyps than in controls. Differences in the pattern of labelling in colonic crypts were compared by the generation of cumulative labelling distributions. Analysis showed a significant expansion of the proliferative compartment in the colon crypts of those with polyps. It is concluded that in vitro labelling with Brdu provides a useful method for the assessment of mucosal proliferation in subjects at high risk of developing colon cancer. 