Expansions of an intronic GAA repeat reduce the expression of frataxin and cause Friedreich's ataxia (FRDA), an autosomal recessive neurodegenerative disease. Frataxin is a mitochondrial protein, and disruption of a frataxin homolog in yeast results in increased sensitivity to oxidant stress, increased mitochondrial iron and respiration deficiency. These previous data support the hypothesis that FRDA is a disease of mitochondrial oxidative stress, a hypothesis we have tested in cultured cells from FRDA patients. FRDA fibroblasts were hypersensitive to iron stress and significantly more sensitive to hydrogen peroxide than controls. The iron chelator deferoxamine rescued FRDA fibroblasts more than controls from oxidant-induced death, consistent with a role for iron in the differential kinetics of death; however, mean mitochondrial iron content in FRDA fibroblasts was increased by only 40%. Treatment of cells with the intracellular Ca2+chelator BAPTA-AM rescued both FRDA fibroblasts and controls from oxidant-induced death. Treatment with apoptosis inhibitors rescued FRDA but not control fibroblasts from oxidant stress, and staurosporine-induced caspase 3 activity was higher in FRDA fibroblasts, consistent with the possibility that an apoptotic step upstream of caspase 3 is activated in FRDA fibroblasts. These results demonstrate that FRDA fibroblasts are sensitive to oxidant stress, and may be a useful model in which to elucidate the FRDA mechanism and therapeutic strategies.