BACKGROUND: Recent developments regarding the contribution of microRNAs to tumor angiogenesis and the oncogenic effects of microRNAs point to their potential role in breast cancer angiogenesis. Tumor-derived exosomes are considered a rich source of microRNAs that can regulate the function of other cells in the tumor microenvironment, including vascular endothelial cells. This study analyzes the effect of tamoxifen chemotherapy on the expression of a key microRNA, miR-329, and introduces a regulatory link between this microRNA and the KDM1A gene associated with the vascular endothelial growth factor (VEGF) messaging pathway. MATERIALS AND METHODS: MCF-7 breast cancer cells were purchased and cultured in a complete culture medium. These cells were treated with tamoxifen and then their exosomes were extracted from the culture medium. The RNAs of the exosomes were isolated and the expression of miR-329, VEGF, and KDM1A genes in the exosomes was investigated using the real-time polymerase chain reaction (PCR) method. RESULTS: The results of this study showed that tamoxifen treatment increased the expression of miR-329 in exosomes derived from MCF-7 cancer cells. The expression of KDM1A and VEGF genes in drug-treated cell exosomes is downregulated. CONCLUSION: The results of this experiment demonstrated that the treatment of breast cancer cells with tamoxifen reduces the expression of VEGF and KDM1A by increasing miR-329. The treatment therefore reduces angiogenesis, and thus its anti-tumor effects are applied.