Introduction Paraneoplastic neurological syndromes (PNS) refer to a diverse group of nervous system disorders associated with tumors. They are not directly caused by a tumor’s immediate expansion, compression resulting from it, infiltration or anticancer treatment. Their pathomechanism involves damage to the nervous system as a result of the organism’s immune response directed against cancer antigens, and also against antigens occurring physiologically in the nervous system. Onconeural antibodies and cytotoxic T lymphocytes are responsible for the immune response. These conditions are quite rare – fewer than 1 person out of 100 patients suffering from cancer develops PNS. They often precede a direct manifestation of cancer. The best known PNS conditions include: Lambert–Eaton myasthenic syndrome, paraneoplastic cerebellar degeneration, and polyneuropathies. PNS was identified for the first time in 1949. Nowadays, the criteria established in 2004 by Graus et al. have been followed in diagnostics. These define the conditions for arriving at a possible and definite diagnosis based on the presence of a characteristic neurological syndrome, the presence of onconeural antibodies, and detecting neoplasm before or after the manifestation of neurological symptoms. Diagnosis is difficult due to the lack of laboratory markers (onconeural antibodies are present in serum only in 50–60% of cases) or direct cancer symptoms in the first phase of the disease. Aim This paper aimed at recounting the existence of a very rare neurological syndrome, limbic encephalitis, which can develop in the course of a neoplastic disease. Awareness of PNS and detecting neoplasm before its direct clinical manifestation can contribute to a better prognosis for patients. Material and methods This paper is based on a case analysis concerning a patient manifesting symptoms of the limbic system involvement. The medical history has been interpreted, additional examination results have been analyzed, and the developing clinical manifestations have been investigated. Case study The presented patient is a 45-year old man in whose case limbic encephalitis exceeded detecting abdominal cancer by a year. Diagnostic difficulties were caused by a synchronous pituitary microadenoma and hormonal disturbances involving cortisol and prolactin, which suggested Cushing’s syndrome. The main clinical symptoms, intensifying in time during the course of the disease lasting several months included: recent memory disturbances, allopsychic disorientation, depression, and periodical aggression. Results and discussion Conducted serum tests to screen for onconeural antibodies did not confirm their presence. However, a magnetic resonance imaginig (MRI) scan revealed bilateral hippocampal damage. On the basis of a typical clinical syndrome, and cancer identified after a year following the occurrence of the symptoms, a definite, according to Graus’ criterion, diagnosis of PNS was established. Conclusions Awareness and knowledge of PNS contributes to an early diagnosis and detection of cancer. This creates an opportunity to introduce adequate treatment and improves the prognosis for the patient.