Serum sialyl-Tn antigen level was studied in 117 cases of colorectal cancer. The positive rates for serum sialyl-Tn antigen were 23.1% in primary colorectal cancer. There is a significant correlation between the positive rates of serum sialyl-Tn antigen and liver metastases, peritoneal dissemination, or lymph node metastases. The survival rate for patients with sialyl-Tn antigen positive sera was lower than those with sialyl-Tn antigen negative. Among patients who underwent curative operation, patients with sialyl-Tn positive sera gave a recurrence rate of 35.7%, while those with sialyl-Tn negative sera recorded a recurrence rate of 14.7%. There is no correlation between the positive rates of serum sialyl-Tn antigen and DNA ploidy pattern. Serum sialyl-Tn antigen levels may be a useful indicator in projecting the prognosis of colorectal cancer.