Mutation of p53, a tumour-suppressor protein, leads to overexpression of the protein and loss of its tumour-suppressive properties. In some tumours (eg, breast) p53 expression is related to well-known prognostic factors, but findings in colorectal tumours are equivocal. We have used the polyclonal antibody CM1 to investigate nuclear and cytoplasmic p53 expression in colorectal tumours and to assess their relations with prognosis. Of 293 colorectal adenocarcinomas, 71 (24%) showed p53 expression in the nucleus alone, 30 (10%) showed p53 in the cytoplasm alone, and 43 (15%) showed both nuclear and cytoplasmic expression. Nuclear p53 expression showed no relation with survival or Dukes' stage of the tumour. However, the frequency of cytoplasmic expression increased with advancing Dukes' stage (chi 2 for trend 11.18, 1 df, p = 0.0008) and cytoplasmic expression was associated with poor survival (rate ratio 2.3 [95% CI 1.6-3.3], p < 0.0001). Among tumours of Dukes' stage A-C, cytoplasmic expression showed prognostic value independent of nuclear staining, grade of differentiation, and Dukes' stage (2.3 [1.4-3.7], p = 0.0021). We conclude that cytoplasmic expression of p53 may be a useful biological indicator of prognosis in colorectal adenocarcinoma.