A glucose controlled insulin infusion system has been used during and after surgery for organic hyperinsulinism in a 37 year old woman. During surgery the system was used to monitor the blood glucose and, in order to avoid insulin hypersecretion, glucose was not infused. A strict correlation was observed between surgical procedures and metabolic behaviour, suggesting the resection of a microinsulinoma despite negative frozen section histology. After surgery plasma insulin was undetectable and an abnormal glycaemic level was observed during an IV glucose load. To ensure an adequate intake of fluid and calories 10% glucose was infused at a rate of 130 mg/min for 16h and blood glucose was clamped at 6.6 mmol/l by feedback insulin infusion. The gradual recovery of residual pancreatic tissue activity was demonstrated by the progressive decrease in exogenous insulin requirement, and confirmed by the normal glycaemic and insulinaemic response to an IV glucose load 20h after surgery.