['The presence of estrogen receptor (ER) and its therapeutic significance in ovarian borderline tumors (OBT) have not been established. We recently observed a response to tamoxifen therapy given empirically to a patient with unresectable, recurrent serous borderline tumor (SBT). In view of this observation the present study was undertaken to assess ER expression in 51 cases of OBT.', 'ER expression was determined retrospectively, using an immunohistochemical method on formalin-fixed, paraffin-embedded specimens, from 35 cases of SBTs, 6 cases of mucinous mullerian (MMBT), and 10 cases of mucinous intestinal borderline tumors (MIBT). ER was considered positive if > 5% of tumor epithelial cell nuclei were immunostained. Both SBTs and mucinous borderline tumors (MBTs) were included to determine the influence of histologic type on ER expression.', 'The patients ranged in age from 25 to 77 years (median 43 years for SBTs, 36 years for MMBTs, and 37 years for MIBTs). The stage distribution for the SBTs was stage I in 27 patients (77%), stage II in 4 patients (11.5%), and stage III in 4 patients (11.5%). All patients with MBTs were stage I. ER expression was observed in the majority of cases and correlated with histologic type: 94% (33/35) of SBTs and 100% (6/6) of MMBTs were ER positive compared to 0% (0/10) of MIBTs (P < 0.01). In the SBT category the presence of ER did not correlate significantly with stage or age. In addition, ER was positive in all four SBT implants (including one involved lymph node) and two recurrent SBTs analyzed.'] Is estrogen receptor expression a common feature of ovarian borderline tumors?