Estrogen receptor levels were determined at the time of ovariectomy in 71 premenopausal women with advanced breast cancer. Three groups of estrogen receptor concentration were established: less than 10 fmol/mg cytosol protein (group 1), between 10 and 25 fmol/mg cytosol protein (group 2), and more than 25 fmol/mg cytosol protein (group 3). The frequency of clinical response to ovariectomy was low in group 1 (17.4%), high in group 3 (79.4%), and intermediate in group 2 (35.7%). Median survival was poor in group 1 (18.5 months) and better in groups 2 (33.0 months) and 3 (32.5 months). These results were independent of disease stage and neoplastic localization.