Two patients had sulfonylurea-induced factitious hypoglycemia. Both patients demonstrated hyperinsulinism during hypoglycemia suggesting the presence of an insulin-secreting tumor. One patient underwent an exploratory laparotomy with subtotal pancreatectomy before the etiology of the hypoglycemia was discovered. Inboth patients, the diagnosis was made by detecting sulfonylurea agents in blood. A survey of Portland, Ore metropolitan hospitals suggests that factitious hypoglycemia occurs with a frequency similar to the insulinoma syndrome. The biochemical similarity of these disorders and the apparent increasing incidence of factitious hypoglycemia suggests that blood determinations of sulfonylurea agents should be performed prior to exploratory laparotomy for an insulinoma.