Nonalcoholic steatohepatitis (NASH) is considered to be a manifestation of metabolic syndrome. Because prevalence and severity of metabolic syndrome are different according to ages, gender and ethnic group, it is speculated that the clinicopathological features of NASH may also vary in relation to these factors. The present study was performed to clarify the influence of age and gender on the development of Japanese NASH. One hundred 93 biopsy-proven NASH patients (86 women and 107 men) were included in this cross-sectional study. The patients were separately analyzed by generation; a younger group (<55 years old) and an older group (>/=55 years old). These groups were compared for their clinical and histological features. Independent risk factors for advanced fibrosis were also analyzed. Comparison of our younger and older groups showed that older patients had much more advanced fibrosis than the younger ones (advanced fibrosis: 23.8%; youngergroup vs. 54.3%; older group, P < 0.001). Women were predominant in the older group (23.8%; younger group vs. 67.4%; older group, P < 0.001). According to the multivariate analysis for risk factors for advanced fibrosis, age (P = 0.007) and BMI (P = 0.028) were independent predictors of advanced fibrosis in the younger group. In contrast, the absence of hyperlipidemia (P = 0.042) was the only significant independent predictor of advanced fibrosis in the older group. Gender was not a risk factor for the severity of NASH. Clinicians need to be aware of age- and gender-specific differences when assessing the characteristics of NASH, and the findings may be useful for prevention and treatment of this disease.