Sialyl Tn antigen was measured by the immunoradiometric competitive inhibition assay in sera from patients with various gynecologic tumors: 49 uterine myomas, 97 cervical cancers, 42 endometrial cancers, 63 benign ovarian tumors, and 59 ovarian cancers including eight cases of borderline malignancy. Among the patients with uterine malignancies, a few cases showed a sialyl Tn antigen level in excess of the cutoff limit (39 U/mL). In the patients with ovarian cancers, serum sialyl Tn antigen was significantly elevated: clinical stage I, 31%; stage II, 29%; and stage III, 69%. The antigen level also correlated with the effect of treatment. In addition, malignant fluid exhibited a high sialyl Tn antigen level, whereas benign fluid showed no elevation. However, serum sialyl Tn antigen was elevated in 6% of the patients with benign ovarian cysts and in 1.5% of normal volunteers. The current findings demonstrate that the lack of tumor specificity of sialyl Tn antigen limits its diagnostic value in gynecologic malignancies, but that serial measurement of this antigen appears to be useful for monitoring patients and evaluating therapy.