Finasteride which specifically inhibits type ”5a-reductase is extremely effective for andorogenetic alopecia (AGA). Type II 5a-reductase was highly expressed in the dermal papilla cells of the AGA regions on the forehead and converted testosterone into dihydrotestosterone (DHT). DHT has high binding affiniity to the andorogen receptor (AR), which is expressed in AGA dermal papilla cells, thus causing AGA-specific regional hair loss. ln addition, there is a poly glutamine domain in the neighboring N terminal of AR, and the CAG repeats coding this region have genetic polymorphism. lt is thought that the sensitivity of the AR for androgen is affected by these genetic polymorphisms. ln this study, we analyzed the correlation between the therapeutic effect of Finasteride and the number of the CAG repeats in the AR gene. The results showed that there was a better therapeutic effect of finasteride to AGA in cases where the CAG repeats were short. This study suggests that the CAG repeat analysis of the AR gene is an effective AGA diagnostic criteria.