Weight loss and changes in growth are noted in children treated with interferon alpha (IFN-α). The aim of this study was to prospectively determine changes in weight, height, body mass index (BMI), and body composition during and after treatment of children with hepatitis C virus (HCV). Children treated with pegylated interferon alpha-2a (Peg-IFN-α2a) ± ribavirin in the Pediatric Study of Hepatitis C (PEDS-C) trial underwent anthropometric measurements, dual-energy X-ray absorptiometry scan, as well as dietary and activity assessments during and after treatment. One hundred and fourteen (55% male) children, with a mean age of 11 ± 3 years, were randomized, and 107 received treatment for at least 24 weeks. Subjects were divided into three groups according to duration of treatment: 24 (N = 14), 48 (N = 82), or 72 (N = 11) weeks. Decrements of up to 0.50 z score were observed for weight, height, and BMI while on therapy among all groups (P ≤ 0.01, compared to baseline). In the group treated for 48 weeks, 29 (33%) subjects had greater than 0.5-unit decrement in height-for-age z (HAZ) score. Though weight-for-age and BMI z scores returned to baseline after cessation of therapy, mean HAZ score was slower to rebound, still lower than baseline at 96 weeks post-therapy for the long-treatment duration group (P = 0.03) and lower than baseline in most children treated for 48 weeks. Percent body fat, fat-free mass z scores, and triceps skinfold z scores decreased with therapy. Dietary energy intake and levels of physical activity did not change during treatment. Conclusions: Peg-IFN-α2a was associated with significant changes in body weight, linear growth, BMI, and body composition in children. These effects were generally reversible with cessation of therapy, although HAZ scores had not returned to baseline after 2 years of observation in many. Longer term growth data are needed among children treated for chronic HCV. (HEPATOLOGY 2012)