Resistance to chemotherapy is the major obstacle to controlling malignant tumors. To characterize multidrug resistance phenotype in human primary ovarian cancer without chemotherapy, expressions of the mdr1 gene in 52 cases of ovarian cancer (44 common epithelial, 5 nonepithelial, and 3 metastatic cancers) were analyzed by polymerase chain reaction of RNA after reverse transcription. Furthermore, localization of P-glycoprotein, which is encoded by the mdr1 gene, was studied immunohistochemically. Although overall expression of the mdr1 gene was relatively low, its expression level was the highest in well-differentiated cancer tissues. Serous and mucinous adenocarcinomas showed higher levels of expression compared with clear cell and endometrioid carcinomas. P-glycoprotein was positive on luminal surfaces of lining cells of ovarian cancer and on those of inclusion cysts from which epithelial ovarian cancer is considered to develop. Thus, some ovarian cancer cases before chemotherapy are intrinsically multidrug resistant, which can be determined by mdr1 gene expression, and this phenotype should be taken into account for effective chemotherapy of ovarian epithelial carcinomas.