Tamoxifen resistance is a major hurdle in the treatment of estrogen receptor (ER)-positive breast cancer. The mechanisms of tamoxifen resistance are not fully understood although several underlying molecular events have been suggested. Recently, we identified autoantibodies reacting with membrane-associated ERalpha (anti-ERalpha Abs) in sera of breast cancer patients, able to promote tumor growth. Here, we investigated whether anti-ERalpha Abs purified from sera of ER-positive breast cancer patients could contribute to tamoxifen resistance. Anti-ERalpha Abs inhibited tamoxifen-mediated effects on cell cycle and proliferation in MCF-7 cells. Moreover, anti-ERalpha Abs hampered the tamoxifen-mediated reduction of tumor growth in SCID mice xenografted with breast tumor. Notably, simvastatin-mediated disaggregation of lipid rafts, where membrane-associated ERalpha is embedded, restored tamoxifen sensitivity, preventing anti-ERalpha Abs effects. In conclusion, detection of serum anti-ERalpha Abs may help predict tamoxifen resistance and concur to appropriately inform therapeutic decisions concerning hormone therapy in ER-positive breast cancer patients.