Estrogen receptor alpha (ERalpha) is highly regulated through multiple mechanisms including cell signaling, posttranslational modifications, and protein-protein interactions. We have previously identified a K303R ERalpha mutation within the hinge region of ERalpha. This mutation results in an altered posttranslational regulation and increased in vitro growth in the presence of low estrogen concentrations. We sought to determine if cells expressing this mutant ERalpha would display hypersensitive tumor growth in in vivo athymic ovariectomized nude mice. MCF-7 cells, stably expressing the K303R ERalpha, formed tumors in nude mice faster than cells expressing wild-type ERalpha in the presence of low levels of estrogen. When estrogen was withdrawn, all tumors regressed but half of the K303R ERalpha-expressing tumors became estrogen-independent and regrew. We evaluated potential mechanisms for the observed hypersensitivity. The mutant ERalpha did not demonstrate increased estrogen binding affinity, but did exhibit increased interactions with members of the SRC family of coactivators. The mutant ERalpha demonstrated increased levels and occupancy time on the pS2 promoter. In the presence of the K303R ERalpha, the SRC-3 and p300 coactivators also displayed increased levels and time on the pS2 promoter. The K303R ERalpha has, in part, lost critical negative regulation by the F domain. Collectively, these data demonstrate an important role for the K303R ERalpha mutation in hormonal regulation of tumor growth and estrogen-regulated promoter dynamics in human breast cancer.