Somatic mutations of the isocitrate dehydrogenase-1 gene (IDH1), most commonly resulting in replacement of arginine at position 132 by histidine (p.R132H), have been reported for WHO grade II and III diffuse gliomas and secondary glioblastomas. We investigated IDH1/2 mutations in a retrospective series of 165 pediatric brain tumors, including atypical teratoid/rhabdoid tumors (AT/RT) and choroid plexus tumors, which had not previously been investigated. Mutation analysis was performed by use of pyrosequencing and, additionally, data were validated for a cohort of 70 gliomas from among the series by use of the arrayed primer extension technique. We identified one tumor which harbored mutation of IDH1 at codon 132 and no alteration was identified in the matched-germline DNA. No IDH2 mutations were detected. Most noteworthy, the IDH1 mutant tumor was an anaplastic astrocytoma involving the cortex in the left frontal lobe which appeared seven years after radiation treatment for an extensive sellar/suprasellar craniopharyngioma. This anaplastic astrocytoma was regarded as secondary to radiation treatment because it seemed to originate within the irradiation field that received a dose varying from a maximum of 30.6 Gy of 4 MV X-rays down to very few Gy of lower-energy scattered radiation. In this work our observations agree with those in previous reports showing the rarity of IDH1/2 mutations in childhood tumors. The interesting identification of an IDH1 mutation in a radiation-induced secondary malignant glioma raises the likelihood that these types of tumor may develop IDH1/2 mutations. Thus, caution is needed when dealing with these tumors, and further genetic analysis is warranted.