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Introduction & Objective: Breast cancer is the most frequent malignancy among women worldwide. Polymorphisms in Xenobiotic Metabolizing Enzymes (XEMs) and drugs such as P450 (CYP2D6) may increase susceptibility to breast cancer. Little is known about the association of CYP2D6*4 (1894 G to A) polymorphism and susceptibility to breast cancer. This study aimed to investigate the possible relationship of the CYP2D6 *4 gene polymorphism and breast cancer. Materials & Methods: One hundred women with confirmed breast cancer and 100 healthy women were the subject of this study. Subjects were assessed for the gene polymorphism of CYP2D6 *4 by a PCR-RFLP assay at Mashad University of Medical Sciences in 2009. The collected data were analyzed by the SPSS using chi-square test and Fisher exact test. Results: No correlation was found between CYP2D6*4 gene polymorphism and breast cancer (P=0.299). Conclusion: Our results demonstrated that CYP2D6 *4 mutant displays a non-significant increased risk for breast cancer.