Nonalcoholic fatty liver disease (NAFLD) represents a spectrum of clinicopatholologic conditions ranging from steatosis alone to nonalcoholic steatohepatitis (NASH), with varying risks for progression to cirrhosis. Although steatosis alone seems to be nonprogressive, some patients with NASH can progress. This study focuses on the clinical and pathological characteristics of patients with NAFLD associated with the development of histological fibrosis. Patients with an established diagnosis of nonalcoholic fatty liver were identified through our NAFLD database containing extensive clinical, demographic, and laboratory data. Liver biopsy specimens were read blindly by one hepatopathologist using a 19-item pathological protocol and by another hepatopathologist using a second pathological protocol. Clinical and pathological data were matched to the presence of different types of histological fibrosis. Univariate and multivariate analyses helped determine all of the variables independently associated with histological fibrosis. Of 132 NAFLD patients, 21.2% had advanced fibrosis (septal/bridging fibrosis or well-established cirrhosis). Sinusoidal fibrosis was present in 20.3% of patients, whereas perivenular fibrosis was seen in 17.2%. Ballooning degeneration and Mallory bodies were independently associated with both sinusoidal fibrosis and perivenular fibrosis. Aspartate aminotransferase/alanine aminotransferase ratio and ballooning degeneration were also independently associated with periportal-portal fibrosis. We conclude that the presence of hepatocyte injury in NAFLD is associated with fibrosis. These pathological features can be used to establish the pathological criteria for diagnosis of the progressive form of NAFLD or NASH.