The incidence and clinical significance of peritumoral brain edema in pediatric patients is not well understood. The purpose of this study is to clarify the clinical significance of peritumoral brain edema in pediatric patients. Seventy seven pediatric patients (under 15 year old) with brain tumor were studied by MRI. The volume of peritumoral edema and brain tumor were measured by integration of the cross-sectional area on serial MRI. The severity of brain edema was expressed by the ratio of edema volume to tumor volume. The results were compared with that of 408 adult patients with brain tumor. Incidence of the brain edema associated with supratentorial tumors is lower in pediatric group than in adult. Severity of brain edema is also less extent in the pediatric group. Forty two percent of supratentorial gliomas in pediatric group showed peritumoral edema. On the other hand, 63% of adult group showed peritumoral brain edema. In general, pediatric brain tumors rarely associated with marked peritumoral brain edema which observed in the adult group. However, intraventricular or paraventricular tumors with obstructed hydrocephalus showed remarkable brain edema in pediatric group. Low incidence and less severity of peritumoral edema are caused not only by histological bias of tumors, but also by the biological nature of developing brain.