A Zollinger-Ellison syndrome due to an ovarian mucinous cystadenoma of borderline malignancy is reported in a 76-year-old woman. Immunostaining procedures showed reactivity for gastrin in neoplastic cells. After operation, the patient has remained without symptoms for 6 years. This is the fifth case reported in the literature. All of these ovarian tumors were considered mucinous cystadenomas histologically.