We compared plasmapheresis with conventional therapy in 245 patients with the Guillain-Barré syndrome of recent onset. Statistically significant differences, favoring the plasmapheresis group, were found in terms of improvement at 4 weeks, time to improve one clinical grade, time to independent walking, and outcome at 6 months. Plasmapheresis was not effective for all patients, but was particularly effective for patients who received this treatment within 7 days of onset and for patients who required mechanical ventilation after entry into the study. Plasmapheresis appears to be of benefit in patients with Guillain-Barré syndrome of recent onset.