LEP and fatty liver disease: A fatty liver or steatohepatitis is the most common liver disease2 and its incidence has increased recently in Japan,3 especially in those between the ages of 40 and 50 years.4 Although the etiology of fatty liver and nonalcoholic steatohepatitis (NASH) are not completely known, studies indicate that steatohepatitis and fatty liver are related to obesity,3,5-19 dyslipidemia,5,8,9,12-14,16-18,20 type 2 diabetes mellitus,8-11,13,17,20,21 leptin,5,15 and insulin resistance.16-19,20-24 Most of these studies were case-control and cross-sectional studies.