The aim of the present prospective study was to evaluate the clinical value of a new serum tumour marker small intestinal mucin antigen (SIMA) in the diagnosis of colorectal cancer. The serum SIMA values were measured in a prospective series of patients with colorectal cancer (n = 73) and patients with benign gastrointestinal disease (n = 87). SIMA values were determined using two different techniques. The cut-off levels (90 % specificity) determined for each test were 12.0 U/ml for SIMA I (Delphia), 9.8 U/ml for SIMA II (PCA-Delphia), 2.5 ng/ml for CEA, 17 U/ml for CA 50 and 17 U/ml for CA 242. The diagnostic sensitivity of the SIMA I test was 0.27, of the SIMA II test it was 0.19, of the CEA test it was 0.63, of the CA 50 test it was 0.30 and 0.30 for the CA 242 test in detecting colorectal cancer. The correlation coefficients (Pearson's r) in colorectal cancer patients between SIMA I and SIMA II measurements were 0.99, 0.71 between CEA and CA 50, 0.70 between CEA and CA 242 and 0.96 between CA 50 and CA 242 measurements. The correlation coefficients in colorectal cancer patients between other serum markers were non-significant. All marker tests were entered in a multivariate analysis to find the best combination of independent predictors of colorectal cancer. The most important predictor of colorectal cancer was SIMA I. 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 0.33 with a specificity of 0.90 and an efficiency of 0.65. In conclusion, the results indicate that the diagnostic sensitivity of a new tumour marker SIMA is equal to CA50 and CA 242, but inferior to the diagnostic sensitivity of the CEA test. However, multivariate discriminant analysis suggests some diagnostic value for SIMA I test because of its independent discriminant value.