In a series of 71 patients with advanced colorectal cancer treated with biochemically modulated 5-fluorouracil (5-FU) and methotrexate (MTX), we investigated the relationship between the proliferating-cell nuclear antigen (PCNA) (PC10) and p53 (Pab1801) primary-tumor immunohistochemical expression with respect to clinical response and long-term prognosis. Nuclear p53 expression was demonstrated in 44% of samples (any number of positive tumor cells) while all tumors showed a certain degree of PCNA immunostaining. PCNA immunostaining was correlated with histopathologic grade and p53 expression, while p53 was not correlated with any of the parameters considered. The probability of clinical response to biochemically modulated 5-FU was independent of p53 and PCNA expression. p53 expression (all cut-off values) was not associated with short- or long-term clinical prognosis, whereas patients with higher PCNA primary-tumor expression showed longer survival from treatment and survival from diagnosis, according to univariate and multivariate analysis, particularly in the sub-set of colon-cancer patients. We conclude that the clinical response of advanced-colorectal-cancer patients to biochemically modulated 5-FU and MTX cannot be predicted by PCNA and p53 primary-tumor expression, but high PCNA expression appears to be independently related to long-term prognosis.