The incidence of ovarian cancer in Rochester, Minnesota over the 40-year period 1935 through 1974 was determined; and risk factors for epithelial ovarian cancer occurring in Rochester from 1945 to 1974 were examined in 116 patients and 464 controls. Among the characteristics studied, only nulliparity was found to be a significant risk factor--relative risk 1.8. Other suspected risk factors--including hypertension, obesity, age at menopause, prior therapeutic pelvic radiation, and prior exposure to exogenous estrogen--were found not to differ significantly between patients and controls. The ovarian cancer patients were found to have a significantly lower frequency of prior hysterectomy and of unilateral oophorectomy than the control group. Thus out data show that hysterectomy, even when one or both ovaries are preserved, is associated with a lower risk of subsequent ovarian cancer.