A 65-year-old man visited our hospital with complaints of tingling sensation in the distal parts of his extremities and dysuria, which first appeared 2 months before admission. He had no abnormal findings on physical examination. Neurological examination revealed sensory impairment of glove and stocking type, mild motor weakness and muscular atrophy in the proximal parts of arms and legs, and absent tendon reflexes in knees and ankles. Fasciculation was observed on his shoulders and upper extremities, and myokymia on the abdominal wall and bilateral calves. He had hyponatremia, which was proved to be caused by SIADH. Anti-acetylcholine receptor antibody, anti-GM1 ganglioside antibody and anti-galactocerebroside antibody were detected in the serum. Chest X-ray showed mass shadows in the mediastinum, which were confirmed as malignant thymoma by needle biopsy. Orthostatic hypotension, neurogenic bladder and anhidrosis were observed by the autonomic function tests. Lesions responsible for orthostatic hypotension and SIADH were suspected in the afferent fibers from baroreceptors, since an reactive increase of plasma arginine vasopressin to orthostatic hypotension was blunted and reflex hypertension in the cold pressor test was well-preserved, while overshoot in Valsalva's maneuver was absent. It is important that afferent baroreceptor dysfunction may be associated with paraneoplastic neurological syndrome, since lesions in acute autonomic neuropathy are usually in the efferent fibers.