During the period between January 1981 and December 1992, a total of ten patients with pathologically confirmed insulinoma were studied. All patients presented with variable degrees of neuroglycopenic symptoms and Whipple's triad. The ratio of insulin to glucose after an overnight fast was greater than 0.3 in 90% (9/10) of patients. Eight patients who received supervised fasting developed neuroglycopenia with relative hyperinsulinemia within 48 hours of fasting. Four of 8 patients developed hypoglycemia during a 5-hour oral glucose tolerance test (OGTT). Calcium infusion test was more sensitive in patients with an initially higher plasma glucose. The detection rates of various localization studies were 12.5% (1/8) by abdominal ultrasonography, 37.5% (3/8) by abdominal CT scan, 50% (5/10) by selective superior mesenteric and celiac arteriography. Transhepatic portal venous sampling (THPVS) detected insulinomas in 4 of 4 cases. Endoscopic ultrasonography and intraoperative ultrasonography were performed on 1 and 2 cases respectively, and were able to localize the lesions successfully. All patients received surgical treatment including enucleation (n = 2), subtotal pancreatectomy (n = 3) and distal pancreatectomy (n = 5). All patients had single tumors which were all benign islet cell adenomas. The mean size of the tumors was 15.5 +/- 2.0 mm in diameter (range: 8 to 30 mm) and mainly located in the body (50%) and tail (40%), only 1 in the pancreatic head. All symptoms of hypoglycemia subsided after operation. Hyperglycemia was observed in all patients immediately after operation, and most of them resumed normoglycemia within 8 days. However, the 2 patients who had impaired OGTT preoperatively had persistent hyperglycemia after operation and therefore were diagnosed as having diabetes (NIDDM).