In recent years, major advances in the diagnosis and treatment of patients with brain tumors have been seen. Today, evaluation of the central nervous system almost always includes magnetic resonance imaging (MRI). The appearance of a new lesion on the MRI scan of a patient previously treated for a central nervous system (CNS) tumor raises concern for recurrent disease with the need for selection of new, potentially toxic therapy. However, the sensitivity of MRI may allow demonstration of new lesions which are not due to tumor. We now report three patients with medulloblastoma who demonstrated new enhancing lesions on MRI following treatment of their tumors with surgery (3 patients), chemotherapy (2 patients), and radiotherapy (2 patients). Two patients underwent resection of the lesion revealing gliosis. One patient had serial imaging that showed disappearance of the lesions. This suggests that not all new enhancing lesions in previously treated brain tumor patients represent tumor. Histologic proof of a suspicious lesion should be demonstrated prior to initiation of new therapy.