Tumor markers known to date are not sensitive and specific enough to detect malignant tumors. Therefore, attempts to find new markers have led to sialic acid assays in cancer patients. Serum sialic acid, CEA and ESR have been determined in 33 patients with the cancer of the colon. All patients have been divided into four groups, according to TNM cancer staging. Serum sialic acid levels have been increased by 100% of patients in groups I and IV. The most significant correlation was noted between sialic acid levels and ESR. No significant relationship between serum sialic acid and CEA have been noted. No correlation of the colon cancer stage, according to TNM staging, and sialic acid and CEA levels in the peripheral blood has been observed. It seems, however, that serum sialic acid assay may be useful auxiliary technique in the detection and monitoring of patients with colon cancer.