Friedreich ataxia is an inherited disorder caused by decreased expression of frataxin protein. Increasing evidence suggests that this protein might detoxify reactive oxygen species (ROS) by an unknown mechanism. Here we demonstrate that transgenic overexpression of human frataxin increases cellular antioxidant defense via activation of glutathione peroxidase and elevation of reduced thiols, thereby reducing the incidence of malignant transformation induced by ROS, as observed by soft agar assays and tumour formation in nude mice. These findings expand the understanding of antioxidant properties of frataxin, and tentatively suggest a role in the early induction of cancer.