Obesity is now prevailing worldwide, coincident with the increase of hepatic steatosis. Metabolic syndrome with obesity and hypertension, hyperlipidemia, and impaired glucose tolerance is one of the most frequent life-threatening diseases and non-alcoholic steatohepatitis is believed to be a hepatic expression of this syndrome. Non-alcoholic steatohepatitis is prevalent and well characterized in Caucasians, but little is known about non-alcoholic steatohepatitis in Asia–Oceania. Obesity will be a serious social problem in Asia–Oceania in the next two decades and we need to prevent the increase of this syndrome. Therefore, it is extremely important to know about non-alcoholic steatohepatitis based on racial differences, because this syndrome is likely to be a multi-factorial syndrome resulting from different combinations of susceptibility genes superimposed on different environmental factors. Because hepatic steatosis is the first step in the development of not only metabolic syndrome but also non-alcoholic steatohepatitis, the genetic background of Japanese NASH patients was investigated and a measure to assess fatty acid β-oxidation in the liver in vivo was developed.