Results of serial positron emission tomography (PET) in a biopsy-proven case of gliomatosis cerebri (GC) are reported. Computed tomography (CT) with and without contrast failed to detect focal abnormalities, but magnetic resonance (MR) revealed iso-intensity or low-intensity lesions in T1-weighted images and high-intensity lesions in T2-weighted images. Lesions were seen in the left thalamus, right temporal lobe and claustrum, and pons. Radiotherapy remarkably improved clinical and imaging findings. Both before and shortly after radiotherapy, 11C-methionine PET images showed hypermetabolism while 15O-water PET images showed a marked increase in cerebral blood flow in GC lesions. However, 6 months later PET images had remarkably improved, appearing nearly normal.