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Context: Congenital hyperinsulinism (CHI) is the most common cause of persistent hypoglycemia in neonates and infants. In medically unresponsive CHI, subtotal pancreatectomy is performed to achieve euglycemia with consequent diabetes in later life. Sirolimus, a mammalian target of rapamycin (mTOR) inhibitor, has been reported to obviate the need for pancreatectomy, but experience is limited., Objective: We have investigated the efficacy and adverse effect profile of mTOR inhibitors in the treatment of severe CHI., Design, Setting, and Patients: This was an observational review of 10 severe CHI patients treated with mTOR inhibitors, in France and the United Kingdom, with the intention of achieving glycemic control without pancreatectomy. Safety information was recorded., Main Outcome Measure(s): We examined whether mTOR inhibitors achieved glycemic control, fasting tolerance, and weaning of supportive medical therapy., Results: mTOR inhibition achieved euglycemia, fasting tolerance, and reduced medical therapy in only three patients (30%). Triglyceride levels were elevated in five patients (50%). One child required a blood transfusion for anemia, four had stomatitis, two had sepsis, one developed varicella zoster, and two patients developed gut dysmotility in association with exocrine pancreatic insufficiency. In silico analysis of transcriptome arrays from CHI patients revealed no significant association between mTOR signaling and disease. Pancreatic tissue from two patients who did not respond to sirolimus showed no reduction in cell proliferation, further suggesting that mTOR signaling did not down-regulate proliferation in the CHI pancreas., Conclusion: mTOR inhibitor treatment is associated with very limited success and must be used with caution in children with severe CHI.