A 44-year-old woman was admitted with obvious symptoms of hypoglycemia and Whipple's triad during a 6-month period. The glucose level was as low as 32mg/dL and insulin/glucose ratio greater than 0.5 after fasting. Abdominal magnetic resonance imaging (MRI) studies revealed a pancreatic mass at the head, 2cm in diameter that was not suggesting because of hypointensity. Selective angiography and somatostatin-receptor scintigraphy did not reveal insulinoma. During laparotomy the tumor was palpated on the ventral surface of pancreas and intraoperative ultrasound accurately localized it. Doppler ultrasound examination clearly revealed the hyper-vascularity of the tumor, which was suggesting an insulinoma. After the tumor enucleation, blood glucose level increased to normal ranges. Histopathological examination revealed benign, well differentiated neuroendocrine tumor, insulinoma. The postoperative recovery was uneventful and the patient is still symptom free during a follow-up period of 6 months. Doppler ultrasound may be a simple but efficient tool for the differential diagnosis of insuhnoma. If the clinical symptoms and findings suggest clearly an insulinoma, intraoperative Doppler ultrasound examination seems to be a simple but the most sensitive diagnostic method.