Two patients with brain stem gliomas were treated with hyperfractionated radiation therapy (HFR) (7,020 and 7,560 cGy, respectively). Despite initial clinical improvement during irradiation, both patients demonstrated clinical deterioration approximately 3 weeks after completion of radiotherapy. Cranial magnetic resonance imaging (MRI) revealed a progressive increase in distribution of abnormal brain stem signal consistent with either tumor or edema. 18FDG positron emission tomography (PET) was obtained in one patient and demonstrated a hypermetabolic lesion at diagnosis and a hypometabolic lesion at the time of clinical deterioration postirradiation. Management with a tapering dose of dexamethasone alone resulted in marked clinical (both patients) and radiographic (one patient) improvement, allowing reduction or discontinuation of this medication. These results suggest that patients with brain stem tumors demonstrating clinical and radiographic evidence of progressive tumor shortly after completion of HFR should be initially managed conservatively with dexamethasone, since these findings may be manifestations of reversible radiation-related neurotoxicity.