Quercetin has potentially beneficial effects on disease prevention, including cancer. An intriguing issue regarding the mechanisms of action of quercetin is the ability of this drug to modulate estrogen receptor (ER) activities. In a previous study, we demonstrated that quercetin elicited apoptosis through an ERα-dependent mechanism. However, the contribution of ERβ in quercetin-induced apoptosis remains elusive. Here, we report that quercetin, at nutritionally relevant concentrations, mimicked the 17β-estradiol (E2)-induced apoptotic effect in both ERβ1-transfected HeLa and in ERβ1-containing DLD-1 colon cancer cell lines by inducing the activation of p38. p38 activation is responsible for pro-apoptotic activation of caspase-3 and the cleavage of poly(ADP-ribose) polymerase. Notably, no inactivation or downregulation of the survival kinases (i.e., AKT and ERK1/2) or the antiapoptotic protein Bcl-2 was observed after quercetin stimulation. On the contrary, quercetin acted similarly to E2 by increasing the levels of the oncosuppressor protein PTEN and by impeding ERβ-dependent cyclin D1 promoter activity, which subsequently resulted in the transcription of the estrogen-responsive element remaining unchanged. As a whole, these data indicate that quercetin mimics the E2 effects in the presence of ERβ1, thus maintaining its anti-carcinogenic potential. In addition, the quercetin pro-apoptotic action in the presence of ERα may render it as a dual-sided protective agent against E2-related cancer in the reduction of tumour growth in organs that express ERα and/or ERβ. J. Cell. Physiol. 227: 1891–1898, 2012. © 2011 Wiley Periodicals, Inc.