Two patients in whom polyneuropathy was associated with hypothyroidism have been studied clinically and electrophysiologically. Sural nerve biopsy was performed on both patients and the nerve studied by light and electron microscopy. Both patients had symptoms of paraesthesiae and muscle pain and there was distal weakness, sensory impairment and incoordination in both upper and lower limbs. Gait was impaired and tendon reflexes were depressed. Electrophysiological studies demonstrated moderate slowing of motor conduction velocity and absent sensory potentials. Microscopic studies of the sural nerves revealed a loss of myelinated fibers of all diameters but particularly those of large diameter. On teased fibre examination, the predominant abnormality was axonal degeneration and electron microscopy showed degenerating fibres, prominent cluster formations, abnormalities of mitochondria and prominent glycogen deposits within Schwann cells. Quantitative study of unmyelinated fibres indicated a relative increase in fibres of small diameter. It is concluded that the polyneuropathy associated with hypothyroidism is due largely to axonal degeneration.