Using data from a study of 296 patients diagnosed in greater Washington, D.C., from 1978 to 1981 with primary epithelial ovarian cancer and 343 patients hospitalized for other conditions, the authors estimated the rate ratios according to various characteristics of the menopause. Menopause induced by hysterectomy with preservation of both ovaries was associated with a 30 per cent reduction in risk of later development of ovarian cancer. Age at natural menopause was not consistently related to risk. Women who used menopausal estrogens showed a 40 per cent decreased risk.