Background: While peripheral neuropathy secondary to B-vitamin deficiencies (specifically, B1 and B12) after gastric bypass surgery has been reported, little is known about demyelinating polyneuropathy following bariatric surgery and the response to plasmapheresis. Patients: We report on 2 patients who developed hip girdle syndrome/lumbosacral plexopathy 8À12 weeks after gastric bypass surgery with severe proximal lower extremity weakness of unclear etiology. Both patients were female, young (28À31 years old), developed severe lower extremity weakness and paresthesias over 5À10 days, and were unable to stand or ambulate. Both patients had episodes of mild to moderate anorexia after surgery but were eating relatively well prior to the development of symptoms, and neither had evidence of post-surgical emesis. Neurologic exam revealed severe hip girdle and bilateral proximal leg muscle weakness, lower extremity areflexia, and normal upper extremity and distal leg strength. Electrophysiologic testing revealed partial motor conduction block, slowed nerve conduction velocities, and abnormal F waves suggestive of peripheral nerve demyelination. Laboratory tests showed normal CBC, metabolic panel, CPK, vitamin B1 and B12, folate, ESR, and ANA results. Cerebrospinal fluid studies revealed normal protein levels and IgG index, and absence of pleocytosis and oligoclonal proteins. Both patients received a course of plasmapheresis (7 treatments over 11 days) and experienced significant neurologic improvement (both patients were able to ambulate short distances with a walker). Both patients received follow-up intravenous immune globulin (IVIG) treatment (5 day course) and intensive physical therapy after plasma exchange. Conclusion: Patients who develop rapidly progressive hip girdle syndrome/lumbosacral plexopathy following bariatric surgery with electrophysiologic studies consistent with demyelinating polyneuropathy may benefit from plasma exchange. # of RBCX w/o tPA 36.7 27.6 7 w/ tPA 45.9 36.9 39 Peripheral Access Group (n = 3) 54.7 49.4 52