Forty-seven patients with stage III or IV invasive epithelial carcinoma of the ovary underwent primary cytoreductive surgery at UCLA during the five-year period 1974 to 1979. Optimal cytoreduction (defined as largest residual tumor mass 1.5 cm or less in diameter) was achieved in 31 patients (66%), including ten of 14 (71%) who underwent laparotomy and biopsy before referral. Median survival for the suboptimal group was six months, compared with 18 months for patients whose largest residual disease was 0.5 to 1.5 cm, and 40 months if residual nodules were less than 0.5 cm (P less than .001). All patients in the suboptimal group died of disease from 22 months to seven years and four months postoperatively. Given the limited ability of chemotherapy to cure ovarian cancer, and the acceptable morbidity of extended operation, the availability of ideal initial surgical effort for patients with advanced stage disease may be the most important variable in current ovarian cancer care. Optimal cytoreduction is most effective in prolonging survival in patients first seen without clinical ascites or large metastatic disease.