Thyroid oncocytoma is characterized by the presence of oncocytes containing abnormally large numbers of mitochondria. However, the relationship between the abundance of mitochondria and the pathogenesis of the tumors is unknown. Recently, a new cell line, named XTC.UC1, has been derived from a metastasis of thyroid oncocytoma. We have studied the metabolism and the gene expression profile of the mitochondria in XTC.UC1 cells, using B-CPAP cells as controls. There were no signs of mitochondrial respiratory chain defects or uncoupling between the respiratory chain and adenosine triphosphate (ATP) production. In XTC.UC1 cells, mtDNA transcripts were increased more than fivefold than in controls, in parallel with a 3.6-fold increase in mtDNA content. Finally, in spite of the glycolytic metabolism induced by the culture medium, the mitochondria of XTC.UC1 cells possess the phenotype of oncocytic cells with hypertrophic mitochondria, higher respiratory enzyme activity and higher mtDNA content than in controls. XTC.UC1 cells may therefore offer a useful model for investigating the coordination of the nuclear and mitochondrial genomes, in the context of thyroid tumors.