Intratumoral thymidylate synthase (TS) expression and M_r53,000 phosphoprotein (p53) overexpression were studied immunohistochemically in sections from stored paraffin-embedded primary colorectal cancers in 70 patients who had undergone surgery during the years 1987–1990. These cancers were classified according to Dukes’stage A–D, using monoclonal antibodies TS 106 and DO-7. In patients with Dukes’ stage A–C tumors, univariate analyses showed that there was a significant correlation (P = 0.048) between disease-free survival and TS expression and between TS expression and time to death with colorectal cancer (P = 0.038). In patients with Dukes’ stage A–D tumors, overall survival was correlated to TS expression (P = 0.015), Dukes’stage (P < 0.001), and level of tumor differentiation (P = 0.044) but not to p53 overexpression. Patients with low intratumoral TS expression survived significantly longer than patients with high expression. Cox multivariate analysis showed that Dukes’ stage (P < 0.001) and TS expression (P = 0.043) could independently serve as prognostic factors for time to death with colorectal cancer in patients with Dukes’ stage A–D tumors.