Thirty-eight patients with suspected supratentorial gliomas have been investigated with positron emission tomography (PET) using 11C-methionine and computed tomography (CT) before and after intravenous injection of contrast medium. Every examination was performed with the head fixed in a plastic helmet and a baseplate to guarantee that the slices were corresponding in the different examinations and from time to time. After the examinations multiple stereotactic biopsies were performed with the biopsy instrument mounted on the baseplate. Accumulation of methionine was compared with normal brain in 32 cases of tumours and ordinary or decreased accumulation in 4 cases of tumours. In 22 cases with increased accumulation of methionine the extension of the tumour judged by PET corresponded with that of histology. In 5 cases tumour cells were found outside the area with pathologic methionine uptake. In 5 patients there were areas with increased methionine accumulation where no tumour cells were found. In 24 cases PET using methionine was more accurate than CT.