© 2015 American Association for Cancer Research. Current paradigms of carcinogenic risk suggest that genetic, hormonal, and environmental factors influence an individual's predilection for developing metastatic breast cancer. Investigations of tumor latency and metastasis in mice have illustrated differences between inbred strains, but the possibility that mitochondrial genetic inheritance may contribute to such differences in vivo has not been directly tested. In this study, we tested this hypothesis in mitochondrial-nuclear exchange mice we generated, where cohorts shared identical nuclear backgrounds but different mtDNA genomes on the background of the PyMT transgenic mouse model of spontaneous mammary carcinoma. In this setting, we found that primary tumor latency and metastasis segregated with mtDNA, suggesting that mtDNA influences disease progression to a far greater extent than previously appreciated. Our findings prompt further investigation into metabolic differences controlled by mitochondrial process as a basis for understanding tumor development and metastasis in individual subjects. Importantly, differences in mitochondrial DNA are sufficient to fundamentally alter disease course in the PyMT mouse mammary tumor model, suggesting that functional metabolic differences direct early tumor growth and metastatic efficiency.