An account is given of four patients with acute idiopathic polyneuritis, leading within a few days to almost total paralysis. Two of these (cases 3 and 4) began to recover voluntary movement in the limbs in a month or less, and showed complete clinical recovery in three and 10 months respectively. The other two (cases 1 and 2) began to recover proximal limb movements after three months, reached a plateau of recovery in about two years, and never recovered movements in the distal parts of the limbs, which underwent muscular atrophy. Patient 2 died 14 years after the acute illness and was examined post mortem. The difference in recovery is explained by supposing that in patients 3 and 4 the lesions consisted predominantly of segmental demyelination, whereas in patients 1 and 2 there was extensive axon destruction at a proximal level. Recovery in the latter depended upon nerve regeneration, which restored the power of the proximal muscles, but was too slow for effective reinnervation of distal muscles. This explanation is supported by post mortem findings in patient 2. A further observation in patient 2 was of degeneration of the posterior white columns of the spinal cord, which was not due to loss of posterior root fibres. It is believed that in such cases a prognosis as to ultimate recovery of muscle power can be made about a month after the acute phase, according to whether movement has begun to return in the distal parts of the limbs. If recovery does not occur within two years it will not occur at all.