The acetone extract from the rhizomes of Alpinia officinarum was evaluated for activity against 5alpha-reductase which had been prepared from rat prostate. The fraction responsible for the inhibition of the enzyme was purified, analyzed, and the active constituents were identified as four diarylheptanoids, 1,7-diphenylhept-4-en-3-one, dihydroyashabushiketol (1,7-diphenyl-5-hydroxy-3-heptanone), 5-hydroxy-7-(4"-hydroxy-3"-methoxyphenyl)-1-phenyl-3-heptanone and 5-hydroxy-7-(4"-hydroxyphenyl)-1-phenyl-3-heptanone.