A 31-year-old woman gradually developed weakness in the lower extremities and gait disturbance. Subsequently, the patient developed severe constipation and hypertension with tachycardia. Nerve conduction studies revealed demyelinating polyneuropathy. Serum anti-GQ1b IgG antibody was detected. The levels of plasma noradrenaline and dopamine and urinary noradrenaline were elevated. Cardiac [¹²³I] metaiodobenzylguanidine (MIBG) scintigraphy showed a normal H/M rate at the early phase and an elevated washout rate. According to these findings, the patient was diagnosed with Guillain-Barré syndrome with cardiac sympathetic hyperactivity. During convalescence, the plasma and urine catecholamine levels fell within the reference ranges, and MIBG scintigraphy showed a decreased washout rate.