Radionecrosis (RN) in children treated for brain tumors represents a potentially severe long-term complication. Its diagnosis is challenging, since magnetic resonance imaging (MRI) cannot clearly discriminate between RN and tumor recurrence. A retrospective single-center study was undertaken to describe the incidence and clinical course of RN in a cohort of 107 children treated with external radiotherapy (RT) for various brain tumors between 1992 and 2012. During a median follow-up of 4.6 years (range 0.29-20.1 years), RN was implied by suspicious MRI findings in in 5 children (4.7 %), 5-131 months after RT. Suspicion was confirmed histologically (1 patient) or substantiated by FDG positron-emission tomography (FDG-PET, 2 patients) or by FDG-PET and MR spectroscopy (1 patient). Before developing RN, all 5 patients had received cytotoxic chemotherapy in addition to RT. In addition to standard treatment protocols, 2 patients had received further chemotherapy for progression or relapse. Median radiation dose expressed as the biologically equivalent total dose applied in 2 Gy fractions (EQD2) was 51.7 Gy (range 51.0-60.0 Gy). At RN onset, 4 children presented with neurological symptoms. Treatment of RN included resection (n = 1), corticosteroids (n = 2) and a combination of corticosteroids, hyperbaric oxygen (HBO) and bevacizumab (n = 1). One patient with asymptomatic RN was not treated. Complete radiological regression of the lesions was observed in all patients. Clinical symptoms normalized in 3 patients, whereas 2 developed permanent severe neurological deficits. RN represents a severe long-term treatment complication in children with brain tumors. The spectrum of clinical presentation is wide; ranging from asymptomatic lesions to progressive neurological deterioration. FDG-PET and MR spectroscopy may be useful for distinguishing between RN and tumor recurrence. Treatment options in patients with symptomatic RN include conservative management (steroids, HBO, bevacizumab) and surgical resection.