With improved treatment for cerebral gliomas leading to longer survival, late rare complications of radiation therapy on the brain are more frequently observed. The paper presents the results of positron emission tomography (PET) with [11C]methionine in 4 patients with malignant cerebral glioma, in whom the phenomenon of transient diffuse high [11C]methionine uptake in the cerebral cortex was detected during treatment monitoring. The unilateral high [11C]methionine uptake in the posterior portion of the cerebral hemisphere was stereotyped and easy to recognize. Nevertheless high amino acid uptake on PET did not match with structural abnormalities on MRI and was not accompanied by aggravation of neurological symptoms or by epileptic seizures. The results of the follow-up examinations excluded tumor recurrence. The control PET/CT scans showed complete resolution of the abnormal [11C]methionine uptake. The clinical and instrumental similarity of the presented cases allowed to establish the diagnostic criteria for the phenomenon of metabolic pseudoprogression and to compare them with the criteria of the SMART syndrome (stroke-like migraine attacks after radiation therapy). Two cases with SMART syndrome are presented as a comparison group. The similarities and discrepancies of unusual metabolic hyperfunction with previously known phenomena and possible mechanisms for their development are discussed. It is important for clinicians and radiologists to be aware of the characteristic imaging findings of the metabolic pseudoprogression on PET with [11C] methionine to avoid overdiagnosis of the brain tumor recurrence and aggressive treatment approach.