A 48-year-old Chinese woman developed ascending motor paralysis while visiting Japan, leading to tetraplegia and respiratory failure over 2 weeks. The patient's course was complicated by anoxic encephalopathy. Nerve conduction studies revealed a severely decreased amplitude of compound muscle action potentials and a sural nerve biopsy specimen showed findings consistent with axonal-form Guillain–Barré syndrome. An autopsy, excluding the brain, demonstrated small-cell lung cancer that was not detected clinically, axonal-dominant degeneration in the nerve roots and distal peripheral nerves, and the loss of both myelin and axons in the dorsal spinal column. The spinal anterior horn cells were severely decreased and were accompanied by astrocytic reaction in all spinal segments with lymphocytic infiltration. A limited examination of the dorsal root ganglia did not show Nageotte nodules, but the infiltration of T cells was observed. Although the clinical course mimicked axonal-form Guillain–Barré syndrome, the autopsy demonstrated both sensory and motor neuronal involvement, as well as small-cell lung cancer. Although anti-Hu and antiganglioside antibodies were negative in the patient's serum, the paraneoplastic mechanism might have damaged the anterior horn and dorsal root ganglia cells, which subsequently led to secondary axonal degeneration. There has been a report on a case of paraneoplastic subacute motor neuronopathy, but the acute course described here has not been reported before.