Depending on the presence or absence of the estrogen receptor in the cells, breast cancer today is often treated by endocrine therapy (tamoxifen) or chemotherapy, respectively. We present now a new paradigm for breast cancer treatment, taking advantage of concepts in bioorganometallic chemistry. In this way, we have synthesized molecules containing an organometallic moiety (ferrocene), and a biovector (hydroxytamoxifen), yielding compounds which display a new therapeutic spectrum consisting of antiestrogenicity and cytotoxicity. A structure–activity relationship study has shown that a ferrocene group, linked to a para-phenol group by a conjugated spacer, is a necessary motif for strong cytotoxic effects to be observed.