Immunohistochemistry of cyclin/proliferation cell nuclear antigen (PCNA) is an attractive alternative to tumour cell proliferation activity determined by flow cytometry which has been shown to be independently predictive of survival in patients with colorectal carcinoma and to enhance Dukes' classification. Dukes' and Jass' histopathological classifications were determined in 91 patients who had undergone curative resection for cancer of the colon (n = 51) or rectum (n = 40) and followed up for a minimum of 10 years. PCNA immunohistochemistry was possible in 79 tumours. Univariate analysis revealed that Jass' (P < 0.0001) and Dukes' classifications (P < 0.0002) were powerful predictors of survival but that the PCNA index had little prognostic power (P = 0.4). Multivariate analysis of both classifications showed similar predictive power and the PCNA index improved the prediction of survival when used with either classification for patients with colon cancer (chi 2 = 5.3, 1 d.f., P = 0.02 for each combination). The PCNA index, however, was not predictive for rectal cancer. Patients with the lowest PCNA index had the worst prognosis.