A 54-year-old woman presented with extremely fluctuating and symptomatic blood glucose levels. Very high levels of somatostatin and low levels of insulin, C-peptide, gastric inhibitory (GIP), and glucagon-like peptide-1 (GLP-1) in peripheral blood were constantly present. A benign somatostatinoma was localized by meta-iodobenzyl guanidine iodine 123 (MIBG-I¹²³) scintigraphy and successfully removed encapsulated in an ovarian teratoma. The patient made a complete recovery. The case described is unique with regard to clinical symptomatology and anatomic localization of the tumor.