In the course of our search for natural estrogenic compounds from medicinal plants, we found that the methanolic extract from the roots of Moghania philippinensis (Fabaceae) showed significant effects on the proliferation of MCF-7 cells (human breast cancer) and induction of β-galactosidase activity in a yeast two-hybrid assay. Through estrogenic activity-guided fractionation, we isolated several active flavonoids including prenylated ones. The CHCl₃ fraction and its new constituent, 8-(1,1-dimethylallyl)genistein (9), appreciably increased the uterine weight in ovariectomized rats when administered orally for 14 consecutive days, in which compound 9 showed stronger estrogenic activity than genistein. Antiestrogenic activities were also examined based on the inhibition of MCF-7 cell proliferation and β-galactosidase activity in the yeast two-hybrid assay, mediated by 17β-estradiol. 5,7,3′,4′-Tetrahydroxy-6,8-diprenylisoflavone (6) showed the strongest antiestrogenic activity.