When patients with cancers are treated with chemotherapeutic agents a long time, some of the cancer cells develop the multidrug resistance (MDR) phenotype. MDR cancer cells are characterized by the overexpression of multidrug resistance1 (MDR1) gene which encodes P-glycoprotein (Pgp), a surface protein of tumor cells that functions to produce an excessive efflux and thereby an insufficient intracellular concentration of chemotherapeutic agents. A variety of studies have sought potent MDR modulators to decrease MDR1 gene expression in cancer cells. Our previous study has shown that curcumin exhibits characteristics of a MDR modulator in KB-V1 multidrug-resistant cells. The aim of this study was to further investigate the effect of curcumin on MDR1 gene expression in patient leukemic cells. The leukemic cells were collected from 78 childhood leukemia patients admitted at Maharaj Nakorn Chiang Mai Hospital, Chiang Mai, Thailand, in the period from July 2003 to February 2005. There were 61 cases of acute lymphoblastic leukemia (ALL), 14 cases of acute myeloblastic leukemia (AML), and 3 cases of chronic myelocytic leukemia (CML). There were 47 males and 31 females ranging from 1 to 15 years old. Bone marrows were collected. The leukemic cells were separated and cultured in the presence or absence of $10{mu}M$ curcumin for 48 hours. MDR1 mRNA levels were determined by RT-PCR. It was found that curcumin reduced MDR1 gene expression in the cells from 33 patients (42%). Curcumin affected the MDR1 gene expression in 5 of 11 relapsed cases (45%), 10 of 26 cases of drug maintenance (38%), 7 of 18 cases of completed treatment (39%), and 11 of 23 cases of new patients (48%). The expression levels of MDR1 gene in leukemic patient cells as compared to that of KB-V1 cells were classified as low level (1-20%) in 5 of 20 cases (25%), medium level (21-60%) in 14 of 32 cases (44%), and high level (61-100%) in 14 of 20 cases (70%). In summary, curcumin decreased MDR1 mRNA level in patient leukemic cells, especially in high level of MDR1 gene groups. Thus, curcumin treatment may provide a lead for clinical treatment of leukemia patients in the future.