The diagnosis of insulinoma on the basis of persistent hypoglycemia requires further confirmation. The insulin suppression test has been used to support this diagnosis prior to surgical intervention. In this study the euglycemic clamp technique was used to compare five control volunteers with four hypoglycemic patients with suspected insulinoma. Insulin was infused over successive two-hour periods at 2, 4, and 8 mU/kg/min. Plasma glucose levels were clamped at 80 mg/dL (4.4 mmol/L) using an artificial pancreas. High insulin levels were measured in all subjects, ranging from 225 +/- 30 microU/mL (1614 +/- 215 pmol/L) to 1018 +/- 239 microU/mL (7304 +/- 1714 pmol/L). Levels of C peptide fell to 0.1 ng/mL (0.028 nmol/L) in control subjects but remained at high levels in the patients. Insulinoma was confirmed on laparotomy in all four patients. In two patients tested after removal of the tumor the results were found to have returned to normal.