Non-alcoholic fatty liver disease, characterized by hepatocyte apoptosis, is distinct in fatty liver and non-alcoholic steatohepatitis, the more severe form. Apoptotic cell death is caspase-dependent and associated with mitochondrial membrane depolarization and cytochrome c release. Adhering to the hypothesis that the exposure of hepatocytes to free fatty acids, resulting in increased ROS production and mitochondrial damage, is balanced by the presence of antioxidant substances, circulating levels of gamma-glutamyl transferase, cytochrome c, triglycerides and unconjugated bilirubin were explored in patients suffering from non-alcoholic fatty liver disease with different severity. One hundred and eighty-six consecutive patients who presented recent ultrasound feature of bright liver without any liver disease of known origin were enrolled, eighty-nine of whom underwent liver biopsy. Forty-five subjects were allocated on the basis of histology in fatty liver group while 44 patients formed the group with non-alcoholic steatohepatitis. A cohort of 27 young, lean, apparently healthy individuals was selected as control group. The levels of gamma-glutamyl transferase were normal or slightly increased, while unconjugated bilirubin concentrations were elevated in all the spectra of non-alcoholic fatty liver disease. Comparing the present results with relevant findings from other studies dealing with diseases characterized by apoptosis, we did not find high circulating levels of cytochrome c in non-alcoholic fatty liver disease. What is more, our patients, categorized as suffering from simple fatty liver or from the more severe non-alcoholic steatohepatitis, had similar levels of cytochrome c and gamma-glutamyl transferase, p=0.19 and 0.11. Serum triglycerides were higher in patients with non-alcoholic fatty liver disease than in the healthy group, p=0.001. These findings likely reflect a balance between oxidative stress and anti-oxidant response rather than a lack of reliability of cytochrome c as a reliable biomarker of mitochondrial damage.