Background: The aim of this study was to assess the utility of F-18-fluoro-2-deoxy-d-glucose (F-18-FDG) positron emission tomography/computed tomography (PET/CT) in assessing bone marrow involvement (BMI) compared with bone marrow biopsy (BMB) in the initial staging of pediatric patients with non-Hodgkin lymphoma (NHL), Hodgkin lymphoma (HL), Ewing sarcoma (ES), and neuroblastoma (NB). Procedure: A total of 94 patients (57 boys, 37 girls, median age 7 y, range 1 to 18 y) with newly diagnosed NHL, HL, ES, and NB between July 2014 and December 2017, who underwent BMB and F-18-FDG PET/CT before chemotherapy were included in this study. There were 36 patients with NHL, 27 HL, 16 ES, and 15 NB. F-18-FDG PET/CT and BMB results were reviewed and compared retrospectively. Findings: Retrospective analysis of data from 94 pediatric patients (57 boys, 37 girls, median age 7 y, range 1 to 18 y) was performed. Of the 94 patients, 29 had BMI on F-18-FDG PET/CT. BMB was positive in 14, negative in 13, and insufficient in 2 of these 29 patients. In 65 patients negative on F-18-FDG PET/CT, BMB was also negative in 54 and insufficient in 7. For the whole group, sensitivity, specificity, and positive and negative predictive values of F-18-FDG PET/CT in detecting bone marrow metastasis at the time of diagnosis were 90.6%, 100%, 100%, and 95.4% and those of BMB were 53.1%, 87.1%, 94.4%, and 80.6%, respectively. Conclusion: Our study demonstrates that F-18-FDG PET/CT predicts BMI better than BMB. F-18-FDG PET/CT may be used at initial staging of pediatric patients with NHL, HL, ES, and NB.