Twenty-five patients with measurable metastatic breast cancer and assays for estrogen receptor (ER) were studied. Of the 16 ER positive patients on anti-estrogen therapy, one had complete disappearance of all tumor for seven months and seven patients had more than 50% reduction in their measurable tumor for an average duration of 8.8 months. Seven other ER positive patients had stabilization of their tumors for an average interval of 8.4 months. Only one of the 16 ER positive patients progressed promptly. Conversely there was only one partial response in the nine ER negative patients and only two ER negative patients had stabilization of disease. Six out of nine ER negative patients progressed promptly. Correlation existed between the duration of response and absolute estrogen receptor level of the tumor. There may be a positive correlation between the response to antiestrogen therapy and response to endocrine ablation but prospective studies must be done to further define the role of antiestrogens in this regard.