Steatosis is a frequent histological finding in chronic hepatitis C infection; however, the pathophysiology of steatosis and its role in disease progression have not been established. We studied 148 consecutive patients with untreated chronic hepatitis C to assess the effect of body mass index, diabetes mellitus, alcohol consumption, hepatic iron content, and viral load on steatosis and hepatic fibrosis. Ninety-one patients (61%) had steatosis: grade 1 (<30% hepatocytes involved) in 61 (41%), grade 2 (30%-70% hepatocytes involved) in 17 (11%), and grade 3 (>70% hepatocytes involved) in 13 (9%). After adjusting for potential confounding variables, a highly significant relationship was found primarily between steatosis and body mass index (P <.0001). The mean (+/-SD) body mass index of patients with no steatosis was 23.9 +/- 4.3 kg/m2, whereas for grade 1 steatosis it was 26.5 +/- 5.1 kg/m2, and for grade 2 and 3 steatosis combined the body mass index was 28.4 +/- 4. 9 kg/m2. Hepatic fibrosis was significantly associated with age (P =. 002). After adjusting for potential confounding variables, including age, hepatic fibrosis was also significantly associated with steatosis (P <.03). There was no significant association between hepatic iron content, alcohol intake, gender, and viral load and steatosis or fibrosis. These findings suggest that increasing body mass index has a role in the pathogenesis of steatosis in chronic hepatitis C and that steatosis may contribute to fibrosis. The association between body mass index and steatosis and fibrosis has important prognostic and therapeutic implications in the management of patients with chronic hepatitis C virus.