The clinical significance of serum sialyl Tn antigen as a tumor marker was evaluated using "STN Otsuka" kits. Results indicated that the antigen was frequently elevated in sera from patients with ovarian cancers (43.1%, 140/325), including serous cystadenocarcinoma (51.6%, 63/122), mucinous cystadenocarcinoma (55.6%, 30/54), endometrioid carcinoma (56.5%, 13/23), and mesonephroid carcinoma (40.0%, 6/15). The positive frequency of sialyl Tn antigen in patients with ovarian carcinoma was less than the frequency of CA 125 (75.5%, 194/257) or sialyl SSEA-1 (47.2% 83/176). However, the false positive rate of sialyl Tn antigen in patients with benign gynecological disorders (3.7%, 15/401) was much lower than the false positive rates of other antigens; such as CA 125 (48.4%, 155/320) and sialyl SSEA-1 (19.2%, 51/266). The serum level of sialyl Tn antigen reflected the clinical activity of the disease quite well in patients with ovarian cancers. The sialyl Tn antigen was concluded to be a useful serum tumor marker for ovarian cancers.