A 65-year-old woman was admitted to our hospital for an investigation of liver dysfunction. She had mild obesity with hyperlipidemia, but no history of alcohol abuse. Other known causes of liver dysfunction, such as viruses, autoimmunity and drug effects, were excluded. The liver histology was consistent with nonalcoholic steatohepatitis (NASH). After diagnosis of NASH, the patient started diet and exercise therapy and, in parallel with weight reduction, her liver function improved. One year after the therapy, a liver biopsy showed that steatosis, necroinflammation and even fibrosis were improved. Hence, here we report a case of NASH in which weight reduction was effective in improving both biochemical and histological findings.