p53 protein expression was evaluated in a series of 204 primary colorectal adenocarcinomas by immunohistochemistry using frozen tissue sections and monoclonal antibody DO-7. Nuclear staining of more than 5% of neoplastic cells was observed in 124 (60.8%) adenocarcinomas, which were classified as p53 positive. p53 immunoreactivity was found to he unrelated to several clinical and pathologic variables, including age and sex of patient, tumor site, tumor stage, grade of differentiation, pattern of growth, degree of peritumoral lymphocytic infiltration, and venous invasion. A strong association was demonstrated between p53 immunostaining and tumor type. Only 4 of 21 mucinous carcinomas examined (19%) were p53 positive. Conversely, 120 of 183 (65.6%) nonmucinous adenocarcinomas showed positive p53 immunostaining (P <.0001). p53 expression also was related to the flow cytometric DNA ploidy pattern, aneuploid carcinomas with DI >1.20 showing higher frequency of p53 overexpression than DNA diploid, and aneuploid tumors with DI < or = 1.20 (P = .0003). No relationship was found between p53 expression and the Ki-67 proliferation index. With respect to the total study population (mean follow-up 33.4 months; range 19-47 months) the duration of overall survival was independent of p53 expression. In the group of 141 patients with stage I, stage II, and stage III disease who had undergone curative resection, positive p53 immunostaining was associated with poorer overall survival (P = .029). Subgroup analysis showed that the reduced survival conferred by p53 overexpression was confined to patients with stage III tumors (P = .027). However, in multivariate analysis, p53 expression failed to demonstrate independent prognostic significance. Our results indicate that immunohistochemical analysis of p53 expression provides valuable information for the understanding of colorectal cancer biology and clinical behavior.