Four cases of severe acute Guillain-Barré syndrome (GBS) characterized by severe axonal degeneration are presented. All had electrically inexcitable motor nerves as early as 4 days after onset. The disease was rapid in onset and the residual disability was severe. Two different types of pathology were seen. Nerve biopsies in 3 cases showed severe axonal degeneration without inflammation or demyelination. Autopsy in one of these cases showed that the dorsal and ventral roots were also significantly affected. These cases illustrate the primary axonal form of GBS. Nerve biopsy in the fourth case at day 15 showed marked inflammation and demyelination with axonal degeneration. Contralateral nerve biopsy at day 75 showed almost complete loss of axons. This case illustrates another type of axonal degeneration, that which occurs secondary to inflammation and demyelination.