Concurrent inhibition of aromatase and steroid sulfatase (STS) may provide a more effective treatment for hormone-dependent breast cancer than monotherapy against individual enzymes, and several dual aromatase-sulfatase inhibitors (DASIs) have been reported. Three aromatase inhibitors with sub-nanomolar potency, better than the benchmark agent letrozole, were designed. To further explore the DASI concept, a new series of letrozole-derived sulfamates and a vorozole-based sulfamate were designed and biologically evaluated in JEG-3 cells to reveal structure-activity relationships. Amongst achiral and racemic compounds, 2-bromo-4-(2-(4-cyanophen-yl)-2-(1H-1,2,4-triazol-1-yl)ethyl)phenyl sulfamate is the most potent DASI (aromatase: IC(50) = 0.87 nM; STS: IC(50) = 593 nM). The enantiomers of the phenolic precursor to this compound were separated by chiral HPLC and their absolute configuration determined by X-ray crystallography. Following conversion to their corresponding sulfamates, the S-(+)-enantiomer was found to inhibit aromatase and sulfatase most potently (aromatase: IC(50) = 0.52 nM; STS: IC(50) = 280 nM). The docking of each enantiomer and other ligands into the aromatase and sulfatase active sites was also investigated.