Objective The prevalence of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) has been increasing in Japan. To determine epidemiological characteristics of NAFLD and suspicious NASH over the past 10 years, changes in the prevalence of ultrasound diagnosed nonalcoholic fatty liver and their ALT abnormality rates were analyzed. Methods The study included 45, 328 men and women who underwent general health check-ups in 1994 and 2004 at PL Tokyo Health Care Center. Changes in the prevalence of ultrasound diagnosed non-alcoholic fatty liver and non-alcoholic fatty liver with ALT abnormality such as suspicious NASH and their body mass index (BMI) were analyzed according to gender and age-specific grouping. Results There was a definitive increase over the past 10 years in the prevalence of ultrasound diagnosed non-alcoholic fatty liver. The prevalence of non-alcoholic fatty liver has risen significantly among men in their 30s to 60s and among women in their 60s to 80s. ALT abnormality rates such as suspicious NASH significantly increased among men in their 30s and 40s and women in their 60s and 70s. The BMI of ultrasound diagnosed non-alcoholic fatty liver showed an increase among men in their 30s and 50s and among women in their 60s and 70s. Increases of BMI were distinct in the age groups in which ALT abnormality rates had risen. Conclusions Over the past 10 years, the prevalence of ultrasound diagnosed non-alcoholic fatty liver and suspicious NASH has significantly increased. The age groups of 30s and 40s in men and 60s and 70s in women were at risk of suspicious NASH with increase of BMI, indicating that changes in age related intravital condition and/or lifestyle habits over lifespan are important.