Replacement of one of the ethyl substituents in diethylstilbestrol by side chains with functional groups converted this potent estrogen into pure antiestrogens with the potential for the treatment of breast cancer. These agents completely suppressed estrogen receptor-mediated gene activation and inhibited the growth of estrogen-sensitive MCF-7 breast cancer cells in submicromolar concentrations. The most potent derivative displayed similar activity as fulvestrant (ICI 182,780) in vitro and in the mouse uterine weight test. Obviously, the stilbene structure can act as a substitute for estradiol in the development of pure estrogen antagonists.