It is often assumed that a parenchymal brain lesion which shows no contrast enhancement on magnetic resonance imaging(MRI) is a"low-grade tumor". We investigated a series of 20 patients with nonenhancing cerebral gliomas ; all underwent stereotactic biopsy or open resection, and histological diagnoses were astrocytoma(n=7), anaplastic astrocytoma(n=7), oligodendroglioma(n=5), and ganglioglioma(n=1). Before surgery, 11 patients underwent [18F] fluorodeoxyglucose positron emission tomography(FDG-PET), and this showed hypermetabolic features in two of four patients with anaplastic astrocytomas and in one of seven with astrocytomas or oligodendrogliomas. Among 17 patients, the mean Ki-67 labeling index was 4.4+/-4.3(range : 0.1-16.7), and the standard error was 1.04. These results indicate that in cerebral gliomas, the enhancing pattern seen on MRI cannot predict the degree of malignancy, and we suggest that even if imaging is consistent with "low-grade glioma", histological verification and close follow-up of clinical course are both important.