Objective: Multidrug resistance(MDR) is one of the most important reasons for treatment failure and recurrence of acute leukemia. Its manifestations are different in children with acute lymphoblastic leukemia(ALL) which may be due to different detection methods. This study was to detect the expression of MDR1 mRNA in bone marrow cells of children with ALL by real-time fluorescence- quantitative reverse transcription polymerase-chain reaction(FQ-RT-PCR), and combine minimal residual desease(MRD) detection by flow cytometry(FCM) and to study their relationship with treatment response and prognosis of ALL. Methods:The MDR1 mRNA levels in bone marrow cells from 67 children with ALL[28 had newly diagnosed disease, 27 had achieved complete remission(CR), 12 recurrent] and 22 children without leukemia were detected by FQ-RT-PCR. MRD was detected by FCM. The patients were observed for 9～ 101 months, with a median of 64 months. Results:Standard curves of human MDR1 and GAPDH genes were constructed successfully. MDR1 mRNA was detected in all children with a positive rate of 100%. The mRNA level of MDR1 was similar among the newly diagnosed ALL group, CR group, and control group(P > 0.05), but significantly higher in the recurrence group than that in newly diagnosed disease group and control group(0.50±0.55 vs. 0.09±0.26 and 0.12±0.23, P < 0.05). 54 ALL patients were followed up, and it was found that MDR1 mRNA level was significantly higher in ALL patients within 3 years duration than that of ALL patients with 3～ 6 years and over 6 years duration(0.63 ± 0.56 vs. 0.11 ± 0.12 and 0.04 ± 0.06, P < 0.01). For the 28 children with newly diagnosed disease, the MDR1 mRNA level was similar between WBC > 50×109 group and WBC<50×109 group(P > 0.05). In the 33 CR patients, the MDR1 mRNA level was significantly higher in MRD>103 group than that in MRD<103 group(0.39±0.47 vs. 0.03± 0.03, P < 0.05). Conclusion:The sensitivity and specificity of FQ-RT-PCR in detecting MDR1 mRNA in bone marrow cells of children with ALL patients are high. MDR1 mRNA is expressed in children with and without leukemia. MDR1 mRNA is highly expressed in the CR ALL patients with high MRD, recurrence and short duration(within 3 years). Monitoring MRD and the MDR1 mRNA level might be helpful for individual treatment. Objective: Multidrug resistance (MDR) is one of the most important reasons for treatment failure and recurrence of acute leukemia. Its manifestations are different in children with acute lymphoblastic leukemia (ALL) which may be due to different detection methods. the expression of MDR1 mRNA in bone marrow cells of children with ALL by real-time fluorescence-quantitative reverse transcription polymerase-chain reaction (FQ-RT-PCR), and combine minimal residual desease (MRD) detection by flow cytometry to study their relationship with treatment response and prognosis of ALL. Methods: The MDR1 mRNA levels in bone marrow cells from 67 children with ALL [28 had newly diagnosed disease, 27 had achieved complete remission (CR), 12 recurrent] and 22 children without MRD was detected by FCM. The patients were observed for 9 ~ 101 months, with a median of 64 months. Results: Standard curves of human MDR1 and GAPDH genes were constr The mRNA level of MDR1 was similar among the newly diagnosed ALL group, CR group, and control group (P> 0.05), but significantly higher in the recurrence group than that in newly diagnosed disease group and control group (0.50 ± 0.55 vs. 0.09 ± 0.26 and 0.12 ± 0.23, P <0.05). 54 ALL patients were followed up, and it was found that MDR1 mRNA level was significantly higher in ALL patients within 3 years duration than that of ALL patients with 3 to 6 years and over 6 years duration (0.63 ± 0.56 vs. 0.11 ± 0.12 and 0.04 ± 0.06, P <0.01). For the 28 children with newly diagnosed disease, the MDR1 The level of mRNA level was significantly higher in MRD> 103 group than that in MRD <103 group (P> 0.05). In the 33 CR patients, the MDR1 mRNA level was similar to that of WBC> 50 × 109 group and WBC <50 × 109 group 0.39 ± 0.47 vs. 0.03 ± 0.03, P <0.05). Conclusion: The sensitivity and specificity of FQ-RT-PCR in detecting MDR1 mRN A in bone marrMDR1 mRNA is highly expressed in the CR ALL patients with high MRD, recurrence and short duration (within 3 years). MDR1 mRNA is highly expressed in children with and without leukemia. MDR1 mRNA is highly expressed in the CR ALL patients with high MRD, mRNA level might be helpful for individual treatment.