Purpose: To evaluate transfusion practices in pediatric oncology and hematopoietic stem cell transplant (HSCT) patients. Methods: This is a multicenter retrospective study of children with oncologic diagnoses treated from 2013-2016 at hospitals that participated in the National Heart Lung and Blood Institute Recipient Epidemiology and Donor Evaluation Study-III (REDS-III). Transfusion practices were evaluated by diagnosis code and pre-transfusion laboratory values. Results: A total of 4766 inpatient encounters of oncology and HSCT patients were evaluated, with 39.3% (95% CI 37.9-40.7%) involving a transfusion. Red blood cells (RBCs) were the most commonly transfused component (32.4%; 95% CI 31.1-33.8%), followed by platelets (22.7%; 95% CI 21.5-23.9%). Patients in the 1 to <6-year old age range were most likely to be transfused and HSCT, acute myelogenous leukemia, and aplastic anemia were the diagnoses most often associated with transfusion. The median hemoglobin (Hb) prior to RBC transfusion was 7.5 g/dL (10-90th percentile: 6.4-8.8 g/dL), with 45.7% of transfusions being given at 7-<8 g/dL. The median platelet count prior to platelet transfusion was 20x109/L (10-90th percentile: 8-51x109/L), and 37.9% of transfusions were given at platelet count of >20-50x109/L. The median international normalized ratio (INR) prior to plasma transfusion was 1.7 (10-90th percentile: 1.3-2.7), and 36.3% of plasma transfusions were given at an INR between >1.4-1.7. Conclusion: Transfusion of blood components is common in hospitalized children with cancer. Relatively high pre-transfusion Hb and platelet values and relatively low INR values prior to transfusion across the studied diagnoses highlight the need for evidence- based practice in this population.