Steroid sulfatases regulate the formation of estrogenic steroids which can support the growth of endocrine-dependent breast tumors. Therefore, the development of potent steroid sulfatase inhibitors could have considerable therapeutic potential. Several such inhibitors have now been developed including estrone 3-O-sulfamate (EMATE, 1), which shows potent active site-directed inhibition. However, EMATE was subsequently shown to be also a potent estrogen. In an attempt to reduce the estrogenicity while retaining the potent sulfatase inhibitory properties associated with this type of molecule, (E)-17-oximino-3-O-sulfamoyl-1,3,5(10)-estratriene (5) (estrone oxime 3-O-sulfamate, OMATE) was synthesized. The X-ray crystal structure of (E)-17-oximino-3-hydroxy-1,3,5(10)-estratriene (4) (estrone oxime) demonstrated the presence of only one geometrical isomer [anti-isomer, (E)]. OMATE potently inhibited estrone sulfatase (E1-STS) activity and was similar to EMATE (>99% inhibition at 0.1 microM in MCF-7 breast cancer cells). It was also evaluated in vivo for its estrogenicity and ability to inhibit sulfatase activity. While it was equipotent with EMATE in vivo as a sulfatase inhibitor, it surprisingly had a stimulatory effect on uterine growth in ovariectomized rats about 1.5-fold greater than that of EMATE. Thus, OMATE possesses potential as a superestrogen and modification at C-17 is identified as a useful route for enhancement of estrogenicity in sulfamate-based estrogens.