A fourteen-month-old girl, who had shown remittent fever frequently from the neonatal period, hypohidrosis, frequent change of face color and self-mutilation of the 1st and 2nd fingers of both hands and tongue in the first months of her life, developed an acute encephalopathy with generalized tonic convulsion outdoors on a sunny hot day. Generalized tonic convulsion subsided within two days, but doll's eye phenomenon, loss of pupillary reaction to light, palpebral myoclonus, and ballismus of arms and legs followed L-dopa showed some effect on the ballismus 1 month after the attack. During the hospital stay, biopsy of sural nerve was performed. Morphometric and ultrastructural studies of the sural nerve demonstrated decreased numbers of unmyelinated and small myelinated fibers. Skin biopsy of the leg revealed sweat glands with no nerve terminals, axons and Schwann cells around them. She was diagnosed as having hereditary sensory and autonomic neuropathy type IV based on the histological and clinical findings. After discharge, bone fracture was found three times without any evidence of trauma. Acute encephalopathy, probably produced in relation to the underlying neuropathy, was considered to be due to heat stroke.