Controversies exist with respect to the mortality of patients undergoing liver transplantation at the extremes of the body mass index (BMI). For pediatric liver transplantation, weight is usually the only factor considered in survival analysis. A review of the United Network for Organ Sharing database (1987-2007) revealed 9701 pediatric patients (<18 years old) who underwent primary liver transplantation. Patients were stratified into 5 BMI categories established by the World Health Organization according to their Z score, which was based on age, gender, and BMI: À3, À2, 0, þ2, and þ3. The survival rates in these 5 categories were compared with Kaplan-Meier survival curves and log-rank testing. Patients with thinness (Z score ¼ À2) and severe thinness (Z score ¼ À3) had significantly (P < 0.0001) lower survival at 1 year (84.4%) versus the survival (88.7%) of the normal and overweight groups (Z score ¼ 0 and Z score ¼ þ 2, respectively). For patients with obesity (Z score ¼ þ3), there was no significant difference in survival early after transplantation, but their mortality gradually increased in the later years after transplantation. By 12 years after liver transplantation, the obese group had significantly (P ¼ 0.04) lower survival (72%) than the normal and overweight groups (77%). In conclusion, liver transplantation holds increased risk for obese pediatric patients. Thin pediatric patients experience early mortality after liver transplantation, and obese pediatric patients experience late mortality after liver transplantation. Transplant management can be modified to optimize the care of these patients.