The efficacy of sialyl SSEA-1 antigen (SLX), a tumor-associated carbohydrate antigen, as a test for gynecological cancer was investigated. The test was found to be positive in 64.5% of all patients with ovarian cancers; this rate is lower than that obtained with CA 125. On the other hand, relatively few false-positive results were observed. Tests were false-positive in 25.0% of patients with endometrial cysts; 25.0% of women in the first trimester of pregnancy and 0.0% of menstruating woman had false-positive results. These percentages were lower than those for CA 125. It is concluded that SLX is a tumor marker with inferior sensitivity and high specificity, compared with CA 125. Since positive tests with SLX in patients with ovarian cancer mostly overlapped the positive tests for CA 125, the usefulness of a combination assay was considered to be low. The SLX test was positive in 18.6 and 25.0% of patients with cervical cancer and endometrial cancer, respectively, and it was concluded that SLX is useless as a serum tumor marker for uterine cancer.