This is a case report of a 43-year-old housewife who developed acute polyneuritis as a terminal complication of chronic pyelonephritis. Death occurred from respiratory failure 18 days after the onset of neurological signs. The principal clinical findings were uraemia and a profound symmetrical weakness of the proximal muscles of the upper and lower limbs. There was also a moderate symmetrical weakness of the distal muscles of the upper and lower limbs. Limb reflexes, apart from the biceps jerks, were absent. There were no sensory abnormalities. Post mortem examination revealed chronic pyelonephritis with secondary hyperparathyroidism and osteitis fibrosa. At all levels of the spinal cord many motor neurones showed chromatolysis. There was degeneration of the terminal arborisations of the intramuscular nerves in distal limb muscles. There were no histological abnormalities in the spinal nerve roots and dorsal root ganglia nor in the extramuscular regions of the peripheral nerves. There was no histological evidence of a myopathy. It is suggested that the abnormalities in the intramuscular nerve fibres are due to a dying-back process of the spinal motor neurones, rather than to changes in the Schwann cells. The nature of the primary defect in the spinal motor neurones responsible for the dying-back phenomenon of the intramuscular nerve fibres is not known.