In 1964, a prospective study was initiated of prophylactic oophorectomy in operable instances of carcinoma of the breast in patients who showed evidence of ovarian activity. Patients were randomized to a control group and a group treated by surgical castration. By Dec. 1979, 359 patients were evaluable at 5 yr and 240 patients at 10 yr. The overall results of the oophorectomy series were superior to that for those of the control group. Patients who had cancer confined to the breast showed no significant benefit from oophorectomy. When 1-3 axillary nodes were involved, women < 50 yr of age benefited significantly from prophylactic oophorectomy, in relapse-free status at 5 yr and in survival and relapse-free status at 10 yr. Patients 50 yr of age and older showed no advantage from oophorectomy. There is a place for prophylactic oophorectomy in patients who are < 50 yr of age with operable carcinoma of the breast and with positive axillary nodes.