Intraoperative hormonal measurements have been used successfully to guide the surgical treatment of various endocrine diseases. In this study, we report the results of intraoperative insulin measurement (IIM) in patients with organic hypoglycemia. IIMs were performed during 52 operations in 51 patients. Hyperinsulinism was secondary to a sporadic insulinoma (M= 40), a type I multiple endocrine neoplasia (MEN‐I) (M= 8), an insulin‐secreting carcinoma (M= 1), or pancreatic nesidioblastosis (M= 2). The insulin was measured with a radioimmunologic assay in blood samples simultaneously drawn from a peripheral vein and the portal vein at the beginning of the operation (T1) and 20 minutes after tumor removal (T2). Normoglycemia was achieved after surgery in 50 cases (96%). Systemic and portal insulin levels were normal at T1 in eight patients, precluding any further interpretation of the test. Completeness of surgery was confirmed by normalization of both systemic and portal insulin levels at T2 in 36 patients. In seven cases the systemic or portal insulin levels (or both) remained elevated at T2 despite a favorable outcome after surgery. Failure of the surgical procedure was predicted in two patients by the persistence of high levels of insulin at T2. In patients with initially elevated serum insulin levels, the positive predictive value and the specificity of intraoperative insulin measurement for completeness of surgery were both 100%. The sensitivity was 84%, the negative predictive value 22%, and the accuracy of the test 84%. We concluded that IIM is a simple, highly reliable tool for predicting the completeness of surgery in patients with organic hypoglycemia. IIM appears to be a valuable addendum to the surgical armamentarium against insulinoma especially for patients with atypical causes, such as MEN, insulin‐secreting carcinoma, or pancreatic nesidioblastosis.