The dietary isothiocyanates (ITCs) exhibit strong chemopreventive activities for a variety of neoplasms including breast cancer. However, the molecular mechanisms underlying ITC function in breast cancer cells have not been well established. Here, we found that phenethyl isothiocyanate (PEITC) acted more potently than the 'pure' anti-oestrogen ICI 182,780 to inhibit the growth of oestrogen receptor (ER)(+) breast cancer MCF7 and H3396 cells and ER(-) MDA-MB-231 and SK-BR-3 cells. PEITC reduced the steady state levels of ER-alpha and its novel variant, ER-alpha36 in a dose-and time-dependent manner and inhibited oestrogen-induced activation of the mitogen activated protein kinase/ERK 1/2 signaling pathway. However, ICI 182,780 that is potent in destabilization of ER-alpha protein, failed to down-regulate ER-alpha36. Our results thus demonstrated that PEITC functions as a more potent ER-alpha'disruptor' than the well-known ICI 182,780 to abrogate ER-mediated mitogenic oestrogen signaling in breast cancer cells, which provides a molecular explanation for the strong growth inhibitory activity of ITCs in breast cancer cells, and a rational for further exploration of ITCs as chemopreventive agents for human mammary carcinogenesis.