In most cases, insulinomas have a hypoechoic appearence in the sonography. But about 10% of insulinomas may have a hyperechoic or isoechoic feature. A 58-year-old woman was admitted due to abnormal behavior and intermittent loss of consciousness. Serum glucose and insulin levels were checked and the results revealed a typical insulinoma. An upper abdominal sonography and MRI were performed, but no remarkable findings were detected. An endoscopic ultrasonography determined the presence of a hyperechoic mass between the tail and body of the pancreas. The insulinoma was confirmed the immuno- histochemical staining and electromicroscopy. Conclusion: it is important to keep in mind that more skeptical observation is needed in performing an endoscopic ultrasonography, especially in cases of hyperechoic insulinoma. (Korean J Gastrointest Endosc 19(Suppl. 2): S112∼S117, 1999)