Predictive value of tumor estrogen and progesterone receptor levels in postmenopausal women with advanced breast cancer treated with toremifene. The predictive value of estrogen receptor (ER) concentrations was evaluated in a group of 113 postmenopausal patients with estrogen-receptor-positive (ER greater than 7 fmol/mg protein) advanced breast cancer. In 103 patients, tumors were also sampled for progesterone receptor (PgR) determination. All patients were treated with toremifene, a novel antiestrogen, 60 mg daily. The median ER in 51 responders was 78 fmol/mg protein, and in 62 nonresponders, 51 fmol/mg protein; the median PgR levels were 40 and 37 fmol/mg protein, respectively. The response rate in patients with ER less than 50 fmol/mg protein was 38%, and 51% in the group with ER greater than 50 fmol/mg protein (not significant [NS]). The response rate in patients with PgR less than 10 fmol/mg protein was 42%, and in patients with greater than 10 fmol/mg protein, 44%. The duration of response in patients with ER greater than 50 fmol/mg protein was significantly longer than with lower ER levels (P = 0.002). PgR was not associated with the duration of response. In Cox's multiple regression analysis, ER was an independent prognostic factor (P = 0.005) for response duration. Thus, the ER concentration of tumor tissue predicts the duration of response but not the response rate to toremifene in patients with advanced breast cancer. The PgR status does not predict the response rate or the duration of response. 