Finasteride is not necessarily effective on all of the male pattern baldness (MPB) patients. To know any factor which correlates with the effectiveness of finasteride, the polymorphism of androgen receptor (AR) gene was analyzed. Symptoms of the 488 MPB patients (18-62 y) before and after treatment with total dose of 10 mg or more of finasteride was typed by photographic method. The number of CAG and GGC repeats in AR gene of MPB patients was determined by DNA sequencing. When the number of the triplet repeats (CAG + GGC) was plotted against the degree of symptom improvement after treatment with this drug, a broad correlation between these variables was observed. The smaller the repeat number, the higher the improvement with finasteride. The group of patients with shorter repeat region in AR gene responded better to this drug than that with longer repeat region, although the former patients tended to reveal severe initial symptoms. Determination of such polymorphism is thought to be useful in the drug choice for MPB patients.