It has been controversial whether mtDNA mutations are responsible for oncogenic transformation (normal cells to develop tumors) and for malignant progression (tumor cells to develop metastases). To clarify this issue, we created transmitochondrial cybrids with mtDNA exchanged between mouse tumor cells that express different metastatic phenotypes. The G13997A mutation in the ND6 gene of mtDNA from high-metastatic tumor cells reversibly controlled development of metastases by overproduction of reactive oxygen species (ROS). The mtDNA-mediated reversible control of metastasis reveals a novel function of mtDNA, and suggests that ROS scavengers may be therapeutically effective in suppressing metastasis.