Objective: To investigate the relations of multidrug resistance gene MDR1 polymorphism in breast cancer with its gene expression and chemotherapy-induced hematologic toxicity. Methods: Genomic DNA was extracted from peripheral venous blood samples from 92 patients with breast cancer, and the polymorphism of MDR1 exon 26 (C3435T) was analyzed by PCR-restriction fragment length polymorphism (PCR-RFLP) technique. The MDR1 mRNA expressions in the breast cancer tissues from these 92 patients and the adjacent tissues from 26 of these cases were determined by RT-qPCR method. Furthermore, the relation of C3435T polymorphism with chemotherapy-induced hematologic toxicity was analyzed according to the clinical data of the patients. Results: In the 92 breast cancer patients, the three C3435T genotypes, CC, CT and TT accounted for 21.7% (20/92), 62.0% (57/92) and 16.3% (15/92), respectively. All the breast cancer tissues showed MDR1 mRNA expression of different levels, but the differences among the genotypes had no statistical significance (F=0.173, P=0.841). The expression level of MDR1 mRNA in breast cancer tissues was significantly higher than that in their paired adjacent tissues (3.83±5.27 vs. 1.81±4.42, t=2.522, P=0.018). There was statistical difference in incidence of neutropenia among patients with different C3435T genotypes, in which, the incidence of neutropenia (grade III-IV) in patients with CC genotype (5.0%) was significantly lower than that in patients with CT or TT genotype (26.3% and 46.7%; χ2=8.075, P=0.018, 95%CI=0.017-0.022). There was no significant difference in the incidences of leucopenia, anemia and thrombocytopenia among genotypes (all P>0.05). Conclusion: MDR1 C3435T polymorphism is irrelevant to MDR1 gene expression in patients with breast cancer, and the patients with TT or CT genotype may have an increased risk of developing chemotherapy-induced neutropenia.