Dipyridamole (DPM) is widely used to prevent strokes and vascular thrombosis. Combination therapy of DPM and antimetabolites has shown synergistic anticancer activity. This study investigated the chemopreventive effects of DPM in the mouse mammary tumor virus promoter-driven polyoma middle T oncoprotein metastatic breast cancer model. We also investigated the effects of DPM on gene and miRNA expression. Chemopreventive activity was assessed by comparing the time to onset of palpable lesions, primary tumor growth kinetics, and the number of lung metastases in transgenic mice treated with DPM or vehicle. Gene expression and miRNA expression profiles of mammary tumor tissues were then analyzed using the Affymetrix GeneChip or miRNA 2.0 arrays. Real-time quantitative PCR was used to confirm changes in gene expression. Treatment with DPM beginning at the age of 4 weeks delayed the onset of palpable lesions, delayed tumor progression, and suppressed lung metastasis. Microarray gene expression analysis identified 253 genes differentially expressed between DPM-treated and control mammary tumors. miRNA expression analysis revealed that 53 miRNAs were altered by DPM treatment. The results indicate that DPM has chemoprevention activity against breast cancer tumorigenesis and metastasis in mice. The array analyses provide insights into potential mechanisms of DPM's chemopreventive effects, involving upregulation of several genes and miRNAs known to suppress cancer growth and/or metastasis and downregulation of genes known to promote cancer. Some of these genes have not been previously studied in breast cancer and may serve as novel molecular targets for breast cancer chemoprevention.