Bromocriptine is an effective treatment for most prolactinomas. Estrogen receptor (ER) antagonists are an alternative for treating patients with bromocriptine-resistant prolactinomas (BCRP). Previously, we reported that fulvestrant, a selective ER antagonist, significantly inhibited the proliferation of, and prolactin secretion by, MMQ cells, a prolactin-secreting rat pituitary cell line, an exemplary model for prolactinoma. In this study, we used fulvestrant to block ER alpha expression by MMQ cells and analyzed the expression of beta-catenin and Wnt inhibitory factor-1 (WIF1) to investigate the effects of fulvestrant on the Wnt signaling pathway. In addition, we examined the gene expression of ER alpha, beta-catenin and WIF1 in clinical BCRP specimens to explore the correlation between gender and clinical features. There was no significant difference in beta-catenin expression between fulvestrant-treated cells and untreated cells, whereas WIF1 expression was higher in the treated cells. In clinical BCRP specimens, ER alpha expression was higher (especially in male patients), whereas beta-catenin expression was similar to normal pituitaries. In addition, WIF1 expression was significantly lower in BCRP specimens than in normal pituitaries. The tumor volume was larger in male patients than in female patients. Prolactin concentration was positively correlated with tumor volume, and a positive linear correlation was observed between ER alpha expression and tumor volume. In conclusion, the anti-tumor activity of fulvestrant on MMQ cells seems to be associated with ER alpha and the non-canonical Wnt pathway, and higher ER alpha levels in male patients with BCRP may contribute to the larger tumor volumes observed. Fulvestrant holds promise as a therapeutic agent for BCRP. (C) 2012 Elsevier Ltd. All rights reserved.