Paraneoplastic neurologic syndromes are diagnosed when neurologic symptoms are associated with neoplasm and other causative factors are excluded. They may precede or be simultaneous to various types of neoplasms, mainly malignant. In men up to 45–50 years old the most common cancer causing the paraneoplastic syndrome is testicle tumor, manifesting usually as limbic/brain stem encephalitis and myelitis. Usually effective treatment of underlying neoplasm brings resolution of neurologic symptoms. But corticosteroids and intravenuous immunoglobulins are also used. In the presented case a 37-year-old man was primarily diagnosed and treated for progressive tetraparesis with signs of both upper and lower motor neuron dysfunction, associated with bulbar symptoms. Having various diagnostic procedures performed an atypical form of chronic inflammatory demyelinating polyradiculoneuronopathy was primarily suspected, but eventually a discovery of endodermal sinus tumor in the testicle enabled to state the diagnosis of possible paraneoplastic syndrome. In spite of chemotherapy the patient died shortly after the diagnosis because of infectious complications. Histopathology displayed intense inflammatory changes in the brain stem as well as in cranial nerves and cervical spinal cord. The same immunological process evoked by various pathogenetic factors (infection vs. neoplasm) may cause similar clinical picture and hinder the diagnosis. Most importantly it may delay the proper way of treatment.