Ninety-six patients with primary ovarian cancer were treated at Tsukuba University Hospital between 1984 and 1992. For all of these patients except stage IV, surgical treatment including pelvic and paraaortic lymphadenectomy was employed to confirm the clinical stage exactly. Re-staging laparotomy was performed in cases with an incomplete initial operation. In thirty cases the primary tumors were truly confined to the unilateral ovary (stage Ia). In six patients, though the tumor appeared to be confined to the unilateral ovary macroscopically at the time of the operation, the stage changed to Ic in one case, IIa in one case and IIIc in 4 cases as found by postoperative confirmation. The most common tumor in stage Ia was mucinous cystadenocarcinoma. In 23 patients with stage Ia, histological grading could be obtained. Twenty-one cases were low potential malignancy or grade 1, whereas grade 3 was found in only one case. None, except one case diagnosed as stage Ia, has had a recurrence in spite of the absence of adjunct therapy, but one patient with grade 3 endometrioid carcinoma died of the disease 10 months after staging laparotomy. In conclusion, even though the tumor appears to be confined to one ovary, a staging laparotomy should be conducted in all patients. When the tumor is proved to be limited to only one ovary and is also histologically confirmed to be less than grade 3, it is considered that no further treatment is necessary.