In Brief Severe hypoglycemia characterized by neuroglycopenic symptoms is a recently described and relatively uncommon complication of gastric bypass surgery. It occurs several months to years after surgery and may be distinct from the more commonly encountered dumping syndrome that occurs early in the postoperative course and usually improves with time. Nesidioblastosis has been proposed as a possible underlying mechanism for late postoperative hypoglycemia. This syndrome is distinct from noninsulinoma pancreatogenous hypoglycemia and likely has a multifactorial etiology. It responds variably to nutrition and pharmacological interventions. Partial pancreatectomy and reversal of the bypass have sometimes been used to ameliorate symptoms.