To determine whether electrophysiological features predict endotracheal mechanical ventilation (ETMV) in Guillain-Barré syndrome (GBS). Non-ventilated GBS patients admitted to an ICU underwent standard electrophysiological testing. Endotracheal mechanical ventilation was decided by physicians who were unaware of electrophysiological results. Sixty consecutive patients underwent electrophysiological testing within 17 days of GBS onset; based on Hadden's criteria, 37 (62%) had primary demyelinating, 18 (30%) equivocal and five (8%) axonal disease. Time at electrophysiological testing and proportions of patients treated by plasma exchange and intravenous immunoglobulins were similar in the three groups, whereas primary demyelinating patients had worse results for disability grade and arm grade. The ETMV was required within 20 days of electrophysiological testing in 20 patients, 17 (46%) in the primary demyelinating group, three (17%) in equivocal group and none in the axonal group (P = 0.02). This prospective study suggests that electrophysiological demyelination may predict a need for ETMV in GBS.