In this study, we show that 5alpha-reductase derived from rat fresh liver was inhibited by certain aliphatic free fatty acids. The influences of chain length, unsaturation, oxidation, and esterification on the potency to inhibit 5alpha-reductase activity were studied. Among the fatty acids we tested, inhibitory saturated fatty acids had C12-C16 chains, and the presence of a C==C bond enhanced the inhibitory activity. Esterification and hydroxy compounds were totally inactive. Finally, we tested the prostate cancer cell proliferation effect of free fatty acids. In keeping with the results of the 5alpha-reductase assay, saturated fatty acids with a C12 chain (lauric acid) and unsaturated fatty acids (oleic acid and alpha-linolenic acid) showed a proliferation inhibitory effect on lymph-node carcinoma of the prostate (LNCaP) cells. At the same time, the testosterone-induced prostate-specific antigen (PSA) mRNA expression was down-regulated. These results suggested that fatty acids with 5alpha-reductase inhibitory activity block the conversion of testosterone to 5alpha-dihydrotestosterone (DHT) and then inhibit the proliferation of prostate cancer cells.