Percutaneous transhepatic portal venography (PTP) and selective portal blood sample collection for immunorcactive insulin (IRI) analyses was done in four patients who had clinical evidence of the presence of an insulin secreting tumor, but selective arteriography of the pancreas did not visualize any insulinomas. In all patients the clinical diagnosis was confirmed and the localization of the tumors could be calculated with the aid of the PTP and the IRI values detected in different parts of the portal trees. Because no tumor was found at the operation in two patients, despite careful exploration of the pancreas, blind distal pancreatectomy was performed to the point suggested with the help of the investigations, and insulinomas were found close to the resection lines. In the two other patients the proposed localization of the tumor preoperatively was confirmed. There have been no postoperative hypoglycemic symptoms.