Nonalcoholic steatohepatitis (NASH) is a condition characterized histologically by macrovesicular steatosis and lobular hepatitis with necrosis or ballooning degeneration and/or fibrosis--a picture resembling alcoholic hepatitis, in the absence of alcohol abuse. Most patients with NASH are asymptomatic, and the disease is detected incidentally. The most common signs of NASH are hepatomegaly and laboratory abnormalities, which include a 2-4-fold elevation of serum aminotransferase levels, while other liver function test results are usually normal. Most patients with NASH are obese, many have diabetes mellitus, hypercholesterolemia, or hypertriglyceridemia. NASH has also been associated with a number of metabolic derrangements, conditions, surgical procedures, and drug treatments. The pathogenesis of NASH is poorly understood, but lipid peroxidation and oxidative stress seem to be the leading culprits. The natural history of NASH is unknown, but it seems to be a stable disease in most patients. Still, the progress to cirrhosis is possible. There is no established treatment for NASH. Treatment is usually directed towards optimizing body weight, and pharmacologic agents are mostly experimentally used. Orthotopic liver transplantation is the treatment of choice for end-stage liver disease secondary to NASH.