[en] Positron emission tomography (PET) diagnosis of brain tumors, especially malignant ones, has been performed with ¹⁸ fluorodeoxyglucose (FDG) probe because of their enhanced glucose metabolism and recently L-¹¹C-methionine (MET) is being watched as another probe due to their enhanced metabolism of amino acid and protein. This review describes the utility of MET-PET for diagnosis of the tumor at the early stage and recurrence with presentation of authors' experience. Evaluation of MET-PET images is routinely done semi-quantitatively with standardized uptake value (SUV) at 20-30 min after administration of MET. Presented are examples of MET-PET in neuroglioma for its diagnosis before surgery and of progression afterward, for application to surgical treatment with higher sensitivity than fluorodeoxyglucose (FDG) and with navigating system construction by fusing with MRI image, for therapeutic efficacy evaluation, and for prediction of prognosis. MET-PET is also useful for detection of metastatic brain tumors as well as of pituitary adenoma, meningioma, brain primary malignant lymphoma and atypical germinoma. MET is occasionally accumulated at the lesion of hemorrhage and infarction and has rather short half life (¹¹C, 20 min): its drawbacks. For these, ¹⁸F-fluorothymidine is being developed and thus further progress in diagnosis of brain tumor is expected. (R.T.)