In a search for novel nonsteroidal inhibitors of human prostatic 5α-reductase, we found a new series of indole derivatives that showed potent inhibitory activities for the human enzyme. Among them, 4-[(1-benzyl-1H-indol-5-yl)oxy]-3-chlorobenzoic aicd (2d, YM-32906) showed more potent inhibitory activity than finasteride with an IC₅₀ value of 0.44 nM. 3-Chloro-4-{[1-(4-phenoxybenzyl)-1H-indol-5-yl]oxy}benzoic acid (2m) showed inhibitory activities for both human and rat prostatic 5α-reductase with IC₅₀ values of 2.1 and 73 nM, respectively. The synthesis and structure-activity relationships of these indole derivatives are presented.