Positron emission tomography (PET) with amino acid-based radiopharmaceuticals is considered as an effective method to diagnose continued growth of cerebral gliomas, but the variability of 11C-methionine uptake by brain lesions of different genesis after combined treatment still remains poorly understood. The aim of this study was to explore the information value of 11C-methionine PET in delimitating progression of cerebral gliomas and stable disease and to assess the risk of tumor recurrence at different values of the 11C-methionine uptake index. Material and methods. We performed a retrospective analysis of the results of 11C-methionine PET or PET/CT in 324 patients suspected for continued growth of cerebral tumor based on magnetic resonance imaging (MRI) findings. A quantitative analysis of the results included calculation of the 11C-methionine uptake index (UI). Results. A ROC analysis revealed that the specificity of PET in the diagnosis of continued tumor growth (CTG) was 98%, and the sensitivity was 71% for a UI of more than 1.9. We found that 98% of lesions with a negative level of RP uptake were related to radiation brain lesions (RBLs) or residual tumors combined with radiation pathomorphims. The UI in a range of 1.2―1.6 in 75% of lesions characterized a stable disease, but 25.5% of the lesions represented continued glioma growth. The proportion of recurrences increased to 40% in a UI range of 1.6―1.9, and 95.5% of brain lesions with a UI of more than 1.9 were tumor recurrences. Therefore, high 11C-methionine uptake with the UI above 1.9 in brain lesions characterized by radiological signs of disease progression is a highly specific indicator of CTG; however, the UI may significantly vary during tumor growth, and a substantial fraction of recurrent gliomas may have lower radiopharmaceutical uptake. In the case of borderline UI values, early dynamic control or complementary additional MRI or CT techniques should be used.