PURPOSE: Multidrug resistance mediated by several drug resistant genes impedes the successful outcome of anti-cancer chemotherapy. In this study, we investigated the expressions of drug resistant genes encoding multidrug resistance (MDR1), multidrug resistance-associated protein (MRP), topoisomerase I (Topo I), topoisomerase II g (Topo II a) in narmal volunteers (n=12) in and patients with myeloid leukemia (n=34). Material and Method: We compared the levels of their transcripts in bone matrow mononuclear cells by semiquantitative RT-PCR. The amount of specific transcripts was represented as the optical density ratio of PCR product of target gene to that of B2- microglobulin (MG). Twenty patients of acute myelogenous leukemia (eight in remission state, twelve in refractory) and fourteen patients of chronic myelogenous leukemia (nine in chronic phase and five in blastic crisis) were examined. Twelve normal healthy persons were compared with leukemic patients. RESULTS: The expression levels of all resistant genes in normal volunteers were relatively high as those of AML patients. Regardless of the disease status including remission status of AML (complete remission versus refractory) and the phase of CML (chronic phase versus blastic phase), the expression levels of all resistant genes in patients with CML were significantly lower than in the patients with AML (p < 0.05). Of interest, the patients with refractary AML did not show any statistical difference in comparison with normal controls and even the patients with AML in complete remission. Among the four drug resistant genes, the optical density ratio of MDRl was significantly lower than that of any other genes (p<0.05). Using HL-60 cell line, we compared the changes of various resistant gene expressions before and after differentiation induced by dimethylsulfoxide. The expressions of resistant genes declined in paralle1 with granulocytic differentiation, suggesting that the induction of cell differentiation might make leukemic cells susceptible to chemotherapeutic agents. CONCLUSION: It is impossibble to explain the mechanism of drug resistance by comparing the level of drug resistant gene expression between nonnal subjects and patients with myeloid leukemias. Therefore, we suppose that longitudinal study of drug resistant gene expression is necessary to demonstrate the development of drug resistant during chemotherapy.