For the first time in history, populations in affluent countries may concomitantly indulge in rich food and physical idleness. Various combinations of obesity, diabetes, and hypertriglyceridemia, with insulin resistance as the common feature, cause hepatic steatosis, which can trigger necroinflammation and fibrosis. Patients with "primary" steatohepatitis exhibit ultrastructural mitochondrial lesions, decreased activity of respiratory chain complexes, and have impaired ability to resynthesize ATP after a fructose challenge. Mitochondria play a major role in fat oxidation and energy production but also leak reactive oxygen species (ROS) and are the main cellular source of ROS. In patients with steatosis, mitochondrial ROS may oxidize hepatic fat deposits, as suggested in animal models. Lipid peroxidation products impair the flow of electrons along the respiratory chain, which may cause overreduction of respiratory chain components, further increasing mitochondrial ROS formation and lipid peroxidation. Another vicious circle could involve ROS-induced depletion of antioxidants, impairing ROS inactivation. Blood vitamin E is decreased in some obese children with steatohepatitis, and serum transaminases improve after vitamin E supplementation. Steatohepatitis is also caused by alcohol abuse, drugs, and other causes. In "secondary" steatohepatitis, mitochondrial ROS formation is further increased as the causative disease itself directly increases ROS or first impairs respiration, which secondarily increases mitochondrial ROS formation. This "second hit" could cause more lipid peroxidation, cytokine induction, Fas ligand induction, and fibrogenesis than in primary steatohepatitis.