Nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH) are common in children and adolescents. However, standard histological criteria for pediatric NAFLD and NASH are undeveloped. We reviewed consecutive patients ages 2 to 18 years with biopsy-proven NAFLD diagnosed between 1997 and 2003. Biopsies were evaluated by two pathologists for individual features of steatohepatitis. Agglomerative hierarchical cluster analysis demonstrated two different forms of steatohepatitis. Type 1 was characterized by steatosis, ballooning degeneration, and perisinusoidal fibrosis; type 2 was characterized by steatosis, portal inflammation, and portal fibrosis. The study included 100 children with NAFLD. Simple steatosis was present in 16% of subjects, and advanced fibrosis was present in 8%. Type 1 NASH was present in 17% of subjects, and type 2 NASH was present in 51%. Boys were significantly (P < .01) more likely to have type 2 NASH and less likely to have type 1 NASH than girls. The NASH type differed significantly (P < .001) by race and ethnicity. Type 1 NASH was more common in white children, whereas type 2 NASH was more common in children of Asian, Native American, and Hispanic ethnicity. In cases of advanced fibrosis, the pattern was generally that of type 2 NASH. In conclusion, type 1 and type 2 NASH are distinct subtypes of pediatric NAFLD, and type 2 is the most common pattern in children. NASH subtypes should be considered when interpreting liver biopsies and planning studies of the pathophysiology, genetics, natural history, or response to treatment in pediatric NAFLD. (HEPATOLOGY 2005;42:641–649.)