Objective: To explore the decrease of the efficacy of preoperative chemotherapy decreases with the period of chemotherapy due to multidrug resistance. Methods: We analyzed 18 cases with or without preoperative chemotherapy. Monoclonal antibody was used to detect the expression of P-glycoprotein (P-gp) in Wilms’ tumor. Fifteen Wilms’ tumor cases with complete clinical records and more than five weeks preoperative chemotherapy were followed up by ultrosonography to identify the relationship between the volume of tumor and the period of chemotherapy and multidrug resistance. Results: It was suggested that the expression of multidrug resistance increases after preoperative chemotherapy. The longer the preoperative chemotherapy

the stronger the expression of multidrug resistance. Conclusion: We have found that decrease of efficacy of chemotherapy after four weeks preoperative chemotherapy is due to the increase of multidrug resistance. We recommend that determination of P-gp using immunohistochemical assay can be served as a basis to formulate preoperative and/or postoperative chemotherapy treatment program.