HEADTOME III, a high resolution PET, has been employed using 15O and 18F labelled pharmaceuticals to evaluate histological malignancy of gliomas preoperatively. PET study was applied on eighteen preoperative gliomas including two recurrent cases. Haemocirculatory and metabolic indices of regional cerebral blood flow (rCBF), cerebral blood volume (rCBV), oxygen extraction fraction (rOEF), cerebral metabolic rates for oxygen (rCMRO2) and glucose (rCMRGI) were measured in the viable portion of the tumour, and the contralateral grey and white matter. In the tumour region, rCBF and rCBV were variable and unrelated to grades of tumour malignancy. rCMRO2 and rOEF values reduced significantly (p < 0.01) relative to the contralateral brain tissue. The average rCMRGI value was 3.00 ± 1.06 mg 100 ml–1 min–1 (mean ± SD) for 7 low grade gliomas (grade II), and 5.91 ± 3.61 mg 100 ml–1 min–1 for 11 high-grade gliomas (grade III and IV). These results would support that anaerobic glycolysis increased in the metabolism of gliomas with malignancy. In comparison with normal volunteers, rCBF, rCMRO2, and rCMRGI values in the contralateral grey matter of gliomas were markedly reduced (p < 0.01, p < 0.05, p < 0.01, respectively) possibly due in part to raised intracranial pressure and depressed cerebral functional activity, so that rOEF was increased to a level of approximately 0.5. PET has been a convincing tool to provide satisfactory information on biological characteristics in the diagnosis of gliomas and the pathophysiological state of brain tissue, and should ultimately make it possible to plan an appropriate therapy.