Abstract: Weight loss has been reported in patients treated with peginterferon alpha (Peg-IFN-α) and ribavirin, however, the pathophysiological mechanism of this weight loss is unclear. We prospectively evaluated the nutritional status, body composition and dietary intake of 10 chronic hepatitis C patients treated with Peg-IFN-α and ribavirin, before, at the end of and at 6 months after treatment. Nutritional status and body composition were evaluated by using anthropometric analyses and Bioelectrical Impedance Analysis (BIA) with a body composition analyzer. Body weight, body mass index and total energy intake were significantly (p<0.05) reduced at the end of treatment compared with before treatment, however, at 6 months after treatment, they were recovered to before-treatment levels. In anthropometric analyses, percent Triceps Skinfold Thickness (TSF%) which represents body fat mass was significantly (p<0.05) decreased at the end of treatment compared with before treatment, whereas, percent Arm Muscle Circumference (AMC%) which represents body muscle was not. In the BIA, body fat mass was also significantly (p<0.05) decreased, whereas, skeletal muscle mass was not. There were no significant changes in grip strength and serum albumin level. Decreased TSF% and body fat mass were also recovered to before treatment values at 6 months after treatment. The body weight loss observed during Peg-IFN-α-2b and ribavirin was attributable to body fat mass decrease rather than skeletal muscle mass decrease. Decreased body weight and decreased body fat mass were recovered to before treatment values at 6 months after treatment without body composition change.