A forty seven year old woman with previously undetected goitre, developed thyroid storm in the immediate postoperative period. Forty eight hours after undergoing nephrolithotomy for renal calculus, she was noted to have flaccid areflexic quadriparesis, most marked in the lower limbs. Electrophysiologic studies revealed evidence of an asymmetric, mixed axonal and demyelinating, motor-sensory peripheral neuropathy. Ultrastructure of the sural nerve showed predominant abnormalities in the mitochondria and cytoskeletal elements compatible with thyrotoxicosis. Treatment of thyrotoxicosis coincided with recovery of symptoms. We believe that thyrotoxicosis precipitated an acute polyneuropathy the ultrastructure of which is being described for the first time.