From among a large group of dogs with acute tetraparesis, we identified 10 dogs with a distinct peripheral nerve disorder. Prior to the onset of signs, all of the dogs had been healthy, and none was known to have been exposed to a neurotoxin or raccoon bite. Weakness, with hypoactive or absent segmental reflexes, became progressively worse for 1 to 21 days. Results of electromyography and nerve conduction studies invariably were compatible with a diagnosis of polyneuropathy that predominantly affected proximal nerve segments. Appearance of nerve biopsy specimens and the short time course for functional recovery suggested a demyelinative component to the disorder. The extent of recovery was variable but often rapid and complete in dogs that did not succumb to complications in the early period. Corticosteroid therapy did not demonstrably influence the outcome. This acute idiopathic polyneuropathy in the dog shares many clinical and pathologic features with idiopathic polyradiculoneuritis (Coonhound paralysis).