For a long time, hepatic steatosis was believed to be a benign condition. Only recently, liver steatosis, also termed non-alcoholic fatty liver disease (NAFLD), has gained much interest. In most cases of NAFLD, a condition regarded as the hepatic component of the metabolic syndrome, the enzyme alanine aminotransferase (ALT) is elevated and consequently has been used as a marker for NAFLD. More recently, several cross-sectional and prospective studies have demonstrated associations of this liver enzyme with features of the metabolic syndrome and type 2 diabetes mellitus. This review discusses the biochemical and metabolic properties of ALT, its applicability as a marker of NAFLD and describes its possible role in the pathogenesis of the metabolic syndrome and type 2 diabetes mellitus and subsequent cardiovascular disease. In addition, treatment strategies to ameliorate NAFLD and the associated risks are discussed. Copyright © 2006 John Wiley & Sons, Ltd.