Several attempts have been made to optimize pretransplant risk assessment to improve hematopoietic stem cell transplantation (HSCT) decision-making and to predict post-HSCT outcomes. However, the relevance of pretransplant risk assessment to the pediatric population remains unclear. We report the results of revalidation of the hematopoietic cell transplantation comorbidity index (HCT-CI) in 874 children who received 944 HSCTs for malignant or nonmalignant diseases at a single center. After finding the HCT-CI invalid in our patient population, we proposed a modified pediatric adapted scoring system that captures risk factors (RFs) and comorbidities (CoMs) relevant to pediatrics. Each RF/CoM was assigned an integer weight based on its hazard ratio (HR) for transplant-related mortality (TRM): 0 (HR < 1.2), 1 (1.2 ≥ HR < 1.75), 2 (1.75 ≥ HR < 2.5), and 3 (HR ≥ 2.5). Using these weights, the pediatric adapted risk index (PARI) for HSCT was devised, and patients were divided into 4 risk groups (group 1: without RF/CoM; group 2: score 1-2; group 3: score 3-4; and group 4: score ≥5). There was a linear increase in 2-year TRM from group 1 to 4 (TRM, 6.2% in group 1, 50.9% in group 4). PARI was successfully validated on an internal and external cohort of pediatric patients. Comparing models using c-statistics, PARI was found to have better performance than HCT-CI in predicting 2-year TRM in children, with Akaike and Schwarz Bayesian information criteria values of 1069.245 and 1073.269, respectively, using PARI, vs 1223.158 and 1227.051, respectively, using HCT-CI. We believe that PARI will be a valuable tool enabling better counseling and decision-making for pediatric patients with HSCT.