STS and 17β-HSD1 are attractive targets for the treatment of estrogen-dependent diseases like endometriosis and breast cancer. The simultaneous inhibition of both enzymes appears more promising than blockage of either protein alone. We describe a designed multiple ligand approach resulting in highly potent dual inhibitors. The most interesting compound 9 showed nanomolar IC50 values for both proteins, membrane permeability, and no interference with estrogen receptors. It efficiently reversed E1S- and E1-induced T47D cell proliferation.