A new class of potent, selective, nonsteroidal inhibitors of aromatase have been discovered. The most potent member of this series is fadrozole hydrochloride, CGS 16949 A, 4-(5,6,7,8-tetrahydroimidazo[1,5-alpha]pyridin-5-yl)benzonitrile monohydrochloride, 26a. In addition, the 6,7-dihydropyrrolo[1,2-c]imidazole (21a) and the 6,7,8,9-tetrahydroimidazo[1,5-alpha]azepine (21b) analogues were synthesized and evaluated. CGS 16949 A's ability to selectively inhibit aromatase (IC50 = 4.5 nM) over other cytochrome P-450 enzymes and suppress estrogen production when administered orally make it a suitable candidate to test the potential of an aromatase inhibitor in estrogen-dependent diseases including breast cancer.