Nerve conduction studies (NCS) and antiperipheral nerve myelin antibody (A-PNM Ab) titers were measured serially in 29 patients with Guillain-Barré syndrome (GBS), of whom 21 were treated with plasmapheresis. Data were obtained from 3 to 6 days until 1 to 2 years after onset of symptoms. Within 3 to 6 days, mean NCS were abnormal. They improved some by 1 week and became maximally abnormal by 4 to 8 weeks, during which time A-PNM Ab fell to low levels. In 5 patients plasmapheresed, A-PNM Ab fell and then increased at 4 to 8 weeks, followed by significant deterioration of NCS (P = 0.01) compared with those without antibody rebound at 18 weeks. These results suggest that, in monophasic GBS, there may be two mechanisms of conduction dysfunction such as early paranodal retraction and later demyelination. In some patients plasmapheresed, A-PNM Ab may rebound associated with further conduction dysfunction. These patients may benefit from further plasmapheresis.