Positron emission tomography (PET) using fluorine-18 (18F)-fluorodeoxyglucose (FDG) has been reported to be a powerful diagnostic and prognostic tool in patients with primary brain tumors. This study was undertaken to compare the prognostic value of: (1) visual grading of FDG uptake in the tumor, (2) the absolute glucose metabolic rate of the tumor (TMRglc), (3) the ratio of glucose metabolism between the tumor and whole brain (T/WB) and (4) between the tumor and contralateral cerebellum (T/CBL). Each of these four parameters was correlated with the survival time in 20 patients with malignant cerebral gliomas. Excellent correlation was obtained with visual grading and reasonably good correlation was obtained with T/WB or T/CBL, but TMRglc alone was only a fair prognostic indicator. Thus, visual grading provides a qualitative analysis and T/WB provides a semi-quantitative analysis neither of which requires arterial blood sampling for quantification of absolute metabolic rates for glucose.