Sialyl Lewis-Xi antigen was measured by sandwich radioimmunoassay in sera from patients with various gynecologic tumors: 27 uterine myomas, 117 cervical cancers, 46 endometrial cancers, 54 benign ovarian cysts, and 47 ovarian cancers. Among the patients with uterine malignancies, only a few cases showed serum sialyl Lewis-Xi antigen values in excess of the cutoff limit. On the other hand, among the patients with ovarian cancers, serum sialyl Lewis-Xi antigen was elevated significantly in the following order: clinical stage I (44%), stage II (50%), and stage III (62%). The antigen level also correlated with the effect of treatment. However, serum sialyl Lewis-Xi antigen was elevated in 9% of patients with benign ovarian cysts and in 1.4% of normal volunteers. The lack of tumor specificity of sialyl Lewis-Xi antigen limits its diagnostic value for gynecologic malignancies, but serial measurement of this antigen may be useful in evaluating therapy and monitoring patients.