Objective: To formulate criteria of multidrug resistance (mdr-1) gene expression for predicting chemotherapy response and prognosis. Methods: Using reverse transcription-polymerase chain reaction (RTPCR) assay, the expression of mdr-1 gene in 82 breast cancer samples were detected. Results: The data were treated by statistic analysis system (SAS)-singlevariate analysis. It showed that the level of mdr-1 gene expression clearly deviated from normal to right distribution (P＜0.0001), and thus might be divided by quantiles P50(mdr-1/β 2-MG=0.2) and P75 (mdr-1/β 2-MG=0.6), which were taken as the preliminary criteria for analyzing 56 patients’ chemosensitivity to ADM、 VDS and VCR in vitro and 32 relapsed metastatic patients’ chemotherapy response in vivo, seperately. When mdr-1/β 2-MG＜0.2, the ratios of resistance gradually escalated, but there were about 30%～50% of the cases who showed sensitive to the drugs in vitro and effective to chemotherapy in vivo. When mdr-1/β 2-MG≥0.6, the most of patients showed drug resistance both in vitro and in vivo. Conclusion:According to the above-mentioned results, criteria of evaluating mdr-1 gene expression level was formulated:the mdr-1/β 2-MG＜0.2 (P50) was considered as negative expression, the ratio≥0.2～＜0.6 (P75) was weakly positive expression, ≥0.6 was strongly positive expression. This indicated that different levels of mdr-1 gene expression may reflect objectively drug resistance in vitro and chemotherapy response in vivo.