Galactosyltransferase Associated with Tumor (GAT) was clinically studied in cases of ovarian cancer. When two cut-off levels of GAT were compared, the cut-off level of 16 U/ml (which corresponds to mean + 2SD among healthy females) was found to be more suitable than the cut-off level of 14 U/ml (the level maximizing the diagnostic efficiency between malignant and benign ovarian tumors). When the GAT positive rate was examined for gynecologic tumors, the rate was 5.7% for benign ovarian cyst, 6.6% for endometriosis, 20.5% for cervical cancer, 19.5% for endometrial cancer, and 52.9% for ovarian cancer. The GAT positive rate for different histologic types of ovarian carcinoma was relatively high for each type, e.g., 55.0% for clear cell adenocarcinoma and 66.7% for endometrioid adenocarcinoma. The GAT positive rate increased gradually with the stage of ovarian cancer. In patients with benign diseases, in particular endometriosis, the GAT positive rate was lower than the positive rate with any other simultaneously determined marker (CA602, CA125, CA54/61, CA72-4, STN, and SLX). The GAT level most weakly correlated with the level of any of the other markers assessed. An assay combining GAT with CA602 or CA54/61 resulted in a higher positive rate and a higher diagnostic efficiency, when compared with the assay for GAT alone. The lower positive rate of GAT in endometriosis, when compared with the positive rate of other markers, suggests the usefulness of GAT in distinguishing malignant ovarian tumors from benign ovarian tumors. The use of GAT in a combination assay is expected to overcome the disadvantages of CA602 or CA125.