A 29-year-old woman presented with blurred vision and distal paresthesias. Her initial evaluation revealed severe bilateral optic disc edema with distal lower-extremity sensory and motor deficits and electrodiagnostic evidence of a length-dependent mixed demyelinating and axonal polyneuropathy. The results of routine diagnostic testing, including laboratory tests, magnetic resonance imaging, and lumbar puncture, were nondiagnostic. A targeted biopsy was ultimately required for diagnosis. In this article, we discuss the differential diagnosis and outline the clinical evaluation indicated for a patient presenting with demyelinating polyneuropathy and concurrent papilledema.