Background: Body composition is associated with chemotherapy toxicity (chemotoxicity) in adults with cancer; this association remains unexplored in children with cancer. Methods: Using baseline computed tomography scans of 107 children with Hodgkin lymphoma (n = 45), non-Hodgkin lymphoma (n = 42), or rhabdomyosarcoma (n = 20), this study examined body composition (skeletal muscle index [SMI], skeletal muscle density [SMD], and height-adjusted total adipose tissue [hTAT]) to determine its association with chemotoxicity. Clinical characteristics and chemotoxicities were abstracted from medical records. Primary outcomes included grade 4 or higher hematologic toxicities and grade 3 or higher nonhematologic toxicities within 6 months of the diagnosis. Logistic regression models accounting for repeated measures were constructed to examine the association between body composition indices and chemotoxicities; adjustments were made for age at diagnosis, sex, race/ethnicity, cancer type, risk group, body mass index (measured as a percentile), or body surface area. Results: The median SMI was 41.0 cm2/m2 (range, 25.8-68.6 cm2/m2), the median SMD was 54.1 HU (range, 35-69.4 HU), and the median hTAT was 19.5 cm2/m2 (range, 0-226.7 cm2/m2). Grade 4 or higher hematologic toxicities and grade 3 or higher nonhematologic toxicities were observed in 74.7% and 66.3% of the chemotherapy cycles, respectively. A higher SMD at diagnosis was associated with lower odds of grade 4 or higher hematologic toxicity (odds ratio [OR], 0.90; 95% confidence interval [CI], 0.85-0.97; P =.004). SMI (OR, 0.99; 95% CI, 0.95-1.04; P =.7) and hTAT (OR, 1.00; 95% CI, 0.99-1.01; P =.9) were not associated with hematologic toxicities. Nonhematologic toxicities did not show any association with body composition. Conclusions: The association between low SMD and hematologic toxicities in children with lymphoma or rhabdomyosarcoma could be due to body composition–based biodistribution of chemotherapeutic agents and needs further investigation. Lay Summary: Body composition at cancer diagnosis in children with lymphoma and rhabdomyosarcoma may provide information that could identify those at risk for serious side effects from chemotherapy. Routinely used measures such as body mass index and body surface area show poor correlations with body composition assessed via computed tomography scans.