Steroid 5a-reductase catalyzes the stereoselective reduction of testosterone to dihydrotestosterone (DHT), which is implicated in benign prostatic hyperplasia (BPH), acne, hirsutism, and male pattern baldness. Two distinct isozymes (type 1 and type 2) exist in humans, with type 1 primarily localized in skin and liver and type 2 in prostate. Selective inhibitors of type 1 enzyme could potentially treat skin-related conditions, while combined inhibition with type 2 inhibitors might improve BPH treatment. Preliminary screening showed 4-aza-4-methyl-5a-cholestan-3-one selectively inhibited the type 1 enzyme from human scalp, and an earlier study suggested a 7β-methyl substituent might enhance potency. We report the stereoselective synthesis of a new series of 7β-substituted 4-azacholestan-3-ones via steps including preparation of 7,9-substituted enones, conversion to α,β-unsaturated ketones, oxidative cleavage, cyclization, catalytic hydrogenation, and N-alkylation. Their in vitro activity against human type 1 and type 2 5a-reductases was evaluated. Results showed 7β-substituted 4-azacholestan-3-ones are potent and selective type 1 inhibitors: introduction of a 7β-methyl group (e.g., compound 12a, MK-386) increased type 1 inhibitory potency (IC50 0.9 nM vs 1.7 nM for the parent compound), and N4-methyl azasteroids were the most potent type 1 inhibitors. Azasteroid 12a (MK-386) was selected for additional studies including human clinical trials. In summary, 7β-substituted 4-azacholestan-3-ones are potent and selective type 1 steroid 5a-reductase inhibitors.