BACKGROUND: Although it is known that losing weight has an effect on the treatment of non-alcoholic fatty liver disease, the studies that show how losing weight affects the non-alcoholic fatty liver disease for the normal weight male adults are limited so far. In this study, we set body mass index as criteria and investigated how the weight changes for 4 years makes an impact on the risk of non-alcoholic fatty liver disease for the male adults who have the normal body mass index. METHODS: From January to December of 2004, among the normal weight male adults who had general check-up at the Health Promotion Center of Ulsan University Hospital, 180 people (average age, 47.4 +/- 4.61 years) who were diagnosed with fatty liver through abdominal ultrasonography were included in this study and were observed according to the variety of data and ultrasonography after 4 years (2008). People who had a history of drinking more than 140 g of alcohol per week or who had a past medical history were excluded from the analysis. The weight change of subjects was calculated using the formula 'weight change = weight of 2008 (kg) - weight of 2004 (kg)' and classified into three groups, loss group (< or =-3.0 kg), stable group (-2.9 to 2.9 kg), and gain group (> or =3.0 kg). The odds for disappearance of non-alcoholic fatty liver disease in those three different groups were compared. RESULTS: Among 180 subjects, compared with stable group (67.2%, 121 subjects), loss group (11.7%, 21 subjects) showed 18.37-fold increase in the odds of disappearance of non-alcoholic fatty liver disease (95% confidence interval [CI], 4.34 to 77.80) and gain group (21.1%, 38 subjects) showed 0.28-fold decrease in the odds of disappearance of non-alcoholic fatty liver disease (95% CI, 0.10 to 0.83). CONCLUSION: Even for the normal weight people, losing weight has an effect on the improvement of non-alcoholic fatty liver disease.