Circulating serum sialyl Tn (STN) antigen levels were measured in 89 patients with epithelial ovarian cancer, 157 benign disease, and in 126 healthy controls. Serum antigen levels were increased in 48.3% of patients with ovarian cancer. The false positive rate is significantly low (4.0% in healthy controls and 9.6% in benign disease). The levels of STN antigen were significantly higher in sera of patients with cancer than in those in benign and healthy controls (p less than 0.05). The rise in serum STN antigen levels correlated to the size of the primary tumors. Of the histological type, it is interesting to note the high sensitivity in mucinous-type ovarian cancer. Survival at 1, 2, 3, 4 and 5 years for patients with STN-negative (serum STN levels less than 50 U/ml) versus STN-positive (serum STN levels greater than or equal to 50 U/mol) was 96.2, 92.3, 86.5, 82.7, and 76.9% versus 59.5, 29.7, 18.9, 10.8, and 10.8%, respectively (p less than 0.05). The overall survival probability was worse in patients with STN-positive sera. Percent progression-free survival at 1, 2, 3, 4 and 5 years for patients with STN-negative versus STN-positive was 90.4, 86.5, 76.9, 59.6, and 51.9% versus 35.1, 16.2, 8.1, 8.1, and 5.4%, respectively (p less than 0.05). The overall progression-free period of survival was shorter in patients with STN-positive sera. Multivariate regression analysis revealed that positive STN, stage, PS and histologic grade were the four variables of most importance in predicting survival. These results indicate that a positive STN antigen level in sera is an independent predictor of poor prognosis in ovarian cancer.