Patients 11 years of age and older with low blood sugar (hypoglycemia) probably caused by an insulinoma may be eligible for this study. Candidates will have their hypoglycemia confirmed (with tests done under NIH protocol 91-DK-0066: Diagnosis and Treatment of Hypoglycemia) and will have CT imaging of the abdomen and MRI and ultrasound tests of the liver and pancreas. Patients whose tumors are not found by these studies will undergo arteriography of the pancreas and hepatic (liver) venous sampling.