Proliferating cell nuclear antigen (PCNA) expression was studied by immunohistochemistry on paraffin-embedded sections of 293 primary colorectal adenocarcinomas and 56 corresponding lymph node metastases. PCNA-positive expression was detected in <25% of tumour cells in 172 (59%) cases and in > 25% in 121 (41%) cases. PCNA accumulation was related to over-expression of c-erbB-2 and p53 and tended to be increased in cases with ras over-expression. PCNA expression was identical in primary and corresponding metastases. No significant relationship was observed between PCNA expression and prognosis and other clinico-pathological variables, including grade of differentiation, growth pattern, Dukes' stage, site, age or sex. We conclude that PCNA expression may be related to alterations of oncoproteins but that PCNA itself could not provide additional information for the development of metastasis and prognosis in colorectal adenocarcinoma.