Paraneoplastic neurological syndromes are a group of syndromes characterized by involvement of different parts of the nervous system (brain, spinal cord, peripheral nerves, and/or muscles) in patients with a primary tumor localized outside the nervous system. The term "paraneoplastic" means that neurological syndromes are not caused by the tumor itself but by the body's immune response to the tumor. Namely, the immune system recognizes the tumor as a foreign body, like an intruder in the body, which leads to the activation of humoral (antibodies) and cellular (lymphocytes) immune response. Due to the similarity of tumor antigens with antigens of the nervous system (mimicry antigens), there is a cross-reaction of the immune system and sometimes very serious, collateral damage to the nervous system. The most common neurological paraneoplastic syndromes are presented, such as paraneoplastic limbic encephalitis, cerebral degeneration, encephalomyelitis, encephalitis associated with anti-NMDAR antibodies, stiffness syndrome, opsoclonus-myoclonus / ataxia, Lambert's syndrome-sensory neuropathic neuropathies dermatomyositis, and necrotizing autoimmune myopathy. The presence of different paraneoplastic antibodies is characteristic of certain paraneoplastic syndromes. The case of a patient with paraneoplastic syndrome of a stiff person and breast cancer in whom 5 years have passed from the first manifestations of paraneoplastic syndrome to the detection of malignancy is presented. If the paraneoplastic syndrome is suspected, and especially in the case of positive paraneoplastic antibodies, extensive diagnostic processing should be performed to determine the primary site of the tumor.