Troglitazone has been withdrawn from therapeutic options for diabetes mellitus because of its severe hepatocyte toxicity of unknown pathogenesis. The aim of the present study was to assess both morphologic and functional alterations in the mitochondria of troglitazone-treated hepatocytes. A polarized human hepatocyte cell line, OUMS-29, was used in this study. The mitochondrial volume and the mitochondrial transmembrane potential (DeltaPsi(m)) were examined using flow cytometry with nonylacridine orange (NAO) and rhodamine-123, respectively. An ultrastructural examination of the troglitazone-treated OUMS-29 cells was performed using transmission electron microscopy (TEM). Reactive oxygen species (ROS) production was assessed using flow cytometry with dihydroethidium and 2',7'-dichlorodihydrofluorescein diacetate. A significant increase in the mitochondrial volume of the troglitazone-treated cells was found by the NAO analysis, in comparison with pioglitazone-treated and ciglitazone-treated cells. The increase in volume was due to a marked enlargement in the mitochondria. The markedly enlarged mitochondria with intramitochondrial electron-dense deposits were confirmed on TEM, which showed myelin-like structures, indicating degraded membrane constituents. The troglitazone-treated cells showed a significant decline in the DeltaPsi(m) per unit mitochondrial volume but resulted in no clear cell death. ROS analysis revealed a significant production of hydrogen peroxide in the troglitazone-treated hepatocytes. This production was attenuated using an antioxidant, N-acetyl-L-cysteine. In conclusion, troglitazone caused overproduction of hydrogen peroxide, which deteriorated both mitochondrial membrane structures and mitochondrial function, leading to a possible priming for the severe hepatocyte toxicity.