The prevalence of the metabolic syndrome (MetS), a cluster of cardiovascular 
risk factors associated with obesity and insulin resistance, is dramatically 
increasing in Western and developing countries. This disorder is not only 
associated with a higher risk of appearance of type 2 diabetes and 
cardiovascular events, but impacts on the liver in different ways. Nonalcoholic 
fatty liver disease (NAFLD) is considered the hepatic manifestation of MetS, and 
is characterized by triglyceride accumulation and a variable degree of hepatic 
injury, inflammation, and repair. In the presence of significant hepatocellular 
injury and inflammation, the picture is defined 'nonalcoholic steatohepatitis' 
(NASH), that has the potential to progress to advanced fibrosis and cirrhosis. 
Diagnosis of NASH is based on a liver biopsy, and active search for noninvasive 
tests is ongoing. Progression of steatohepatitis to advanced fibrosis or 
cirrhosis has been shown in at least one third of patients followed with paired 
biopsies. Presence of NASH is associated with lower life expectancy, both due to 
liver-related death and to an increase in cardiovascular events. The appearance 
of NAFLD is mainly dependent on increased flow of fatty acids derived from an 
excess of lipolysis from insulin-resistant adipose tissue. Development of NASH 
is based on lipotoxicity and is influenced by signals derived from outside the 
liver and from intrahepatic activation of inflammatory and fibrogenic pathways. 
The presence of the MetS is also associated with worse outcomes in patients with 
cirrhosis due to any causes, and has complex interactions with hepatitis C virus 
infection. Moreover, MetS poses a higher risk of development of hepatocellular 
carcinoma, not necessarily through the development of NASH-related cirrhosis. In 
conclusion, the presence of metabolic alterations has a severe and multifaceted 
impact on the liver, and is responsible for a higher risk of liver-dependent and 
-independent mortality.