We report a patient with a reversible multifocal encephalopathy that developed 3 days after completion of treatment with intravenous immunoglobulin for Guillain-Barré syndrome. Magnetic resonance imaging showed reversible bioccipital white matter changes, and transcranial Doppler demonstrated transiently increased flow rates in all major cerebral arteries. We conclude that intravenous immunoglobulin therapy caused cerebral arterial vasospasm and a reversible encephalopathy.