In a search for novel nonsteroidal inhibitors of human prostatic 5α-reductase, we found a new series of phenoxybenzoic acid derivatives to be potent human prostatic 5α-reductase inhibitors. Among them, 4-(biphenyl-4-yloxy) benzoic acid derivatives (2n, YM-31758), 2o and 2s showed more potent inhibitory activities than finasteride with IC 50 values of 0.87, 0.67 and 0.56 nM, respectively. The optimized structures for the phenoxybenzoic acid ...