A case of 36-year-old nonmarried virgin woman presenting a giant ovarian serous cyst adenoma weighing 9.5 kg is reported here. Ovarian neoplasms may be divided by origin cell type into three main groups: epithelial, stromal and germ cell. Taken as a group, the epithelial tumors are by far the most common type. The single most common benign ovarian neoplasm is the benign cystic teratoma; however, according to some studies it is serous cyst adenoma. A 36-year-old nonmarried virgin woman was referred to our clinic from a local medical center. When she was seen first at our outpatient clinic, she looked like a 9-month pregnant woman. Her medical history was normal. She had no serious illness or operation before. On abdominal ultrasound, a giant cyst was found which encompassed the whole abdomen. At laparotomy, a giant, totally cystic, vascularized and smooth mass attached to the right ovary was encountered, lying between the symphysis and the xiphoid. Ooferectomy was performed. On the postoperative second day, she was discharged without any problem. Her pathology report disclosed a 35×20×16 cm³ serous cyst adenoma weighing 9.5 kg. This is the largest ovarian cyst that ever reported from our hospital and one of the largest among the reported cases in the literature.