Primary central nervous system lymphoma is a rare neoplasm, exceptionally responsible for developing a frontal lobe syndrome. A 71-years-old patient with primary central nervous system lymphoma had undergone frontotemporal craniotomy for tumor removal. Month later, showing syncopies, disorientation and slurred speech, he was transported to Department of Internal Medicine. Neuroimaging revealed numerous tuberous changes in frontal and parietal lobes and frontotemporal area. During hospitalization, patient exhibited inadequate affect, jocular behaviour, was disoriented in the place and time, and unaware of his health state. He took repeated, uncritical attempts to jump out the window and started to exhibit persecutory delusions and hallucinations. Immediate brain radiotherapy was recommended by the oncology specialists. However, the patient did not consent to the recommended method of treatment due to his lack of recollection of the tumor or the craniotomy. The patient was forcibly transferred to Department of Psychiatry. After five weeks of treatment, the patient's condition improved significantly, and the patient regained memory of the tumor.