Of 299 obese children who visited our obesity clinics, 36 were found to have elevated levels of serum transaminases by routine laboratory examination. Liver biopsy was carried out in 11 children. Based on the criteria of Adler and Schaffner (1979), the biopsy specimens were studied histologically. As a result, fatty liver (Group I) was observed in three patients, fatty hepatitis (Group II) in two, fatty fibrosis (Group III) in five, and fatty cirrhosis (Group IV) in one. The duration of obesity, but not its degree, was considered to be related to the extent of fibrosis. Accordingly, we concluded that the fatty liver of simple obesity in children may progress to liver cirrhosis and that childhood obesity should be treated as early as possible.