Expression of glutathione S-transferase-pi (GST-pi) was examined immunohistochemically in relation to the response to chemotherapy with cisplatin in 61 patients with primary ovarian cancer who had not received any chemotherapy before surgery. GST-pi were detected in the cancer tissues of 33 patients (54.1%). Of 28 clinically measurable lesions after surgery, 10 of the 11 tumors (90.9%) showing positive GST-pi staining were drug resistant (no change or progressive disease), whereas 6 of the 17 tumors (35.3%) showing negative staining were drug resistant. This difference in the response rates of the two groups was significant (P < 0.005). The survival period of patients with GST-pi-positive tumors was also significantly shorter than that of those with GST-pi-negative tumors (P < 0.005). These data strongly suggest that GST-pi expression in tumor cells is related to drug resistance of patients with epithelial ovarian cancer and is a useful marker of a poor prognosis.