Paraneoplastic neurologic syndromes (PNS) are neurologic disorders that can affect any part of the nervous system, occur in association with cancer, and have an immune-mediated mechanism that produces direct damage to the neural tissue. Neurological symptoms frequently precede, in months to years, the symptoms directly attributed to the primary tumor, requiring a high clinical suspicion for adequate investigation. We report the case of a man in his early 80s admitted for an altered level of consciousness, alternating between periods with stupor and drowsiness, short-term waking states and psychomotor agitation, respiratory failure and dysautonomia, resembling a Morvan's syndrome. Anti-leucine-rich glioma-inactivated 1 and anti-contactin-associated protein-like 2 antibodies were both positive and, after exclusion of infectious and autoimmune systemic causes, the possibility of PNS was raised. Screening for the primary tumor was pursued, and an 18F-fluorodeoxyglucose (18F-FDG)/PET showed only an intensely hypermetabolic, apparent parietal thickening of the lower rectum. Due to the frequent association of Morvan's syndrome to thymoma, a review of the CT of the thorax images was requested and a mediastinal image with features of thymoma was identified. PNS treatment and prognosis depend on finding and treating the underlying tumor, with benefits in both resolution of neurological symptoms and in the prognosis of the underlying tumor itself. Therefore, clinicians should be aware of this frequent but underdiagnosed and underreported condition, in order to improve the chances of better outcomes.