The aim of the present prospective study was to evaluate the clinical value ol serum tumour markers CEA, CA 50 and CA 242 in patients with colorectal cancer (n = 138) and patients with benign gastrointestinal disease (n = 104). The cutoff levels (90% specificity) determined for each test were 2.5 ng/ml for CEA, 17 U/ml for CA 50 and 17 U/ml for CA 242. The diagnostic sensitivity of the CEA test was 0.63, that of the CA 50 test was 0.30 and 0.30 for the CA 242 test in detecting colorectal cancer. CEA, CA 50 and CA 242 tests were tested in a multivariate analysis to find the best combination of independent predictors of colorectal cancer. The most important predictor of colorectal cancer was CEA followed by CA 242. In order to calculate the contributions of tumour marker tests, a diagnostic score (DS) was developed. The sensitivity of the DS in detecting colorectal cancer was 0.47 with a specificity of 0.88 and an efficiency of 0.67. On the basis of this study, serum CEA and CA 242 seem to possess diagnostic value in preoperative evaluation of patients with colorectal cancer.