In order to determine the significance of androgen receptor (AR) gene mutations for Japanese prostate cancers, we examined the entire coding region, from exon A to H, in 36 primary lesions. Five in stage A, 12 in stage B, six in stage C and 13 in stage D were subjected to PCR-SSCP analysis for genomic DNA and nucleotide sequencing. Mutations were detected in five samples (14%). Two in stage D and refractory to anti-androgen treatment showed mis-sense mutations. The other three showed changes in the length of the CAG repeat in exon A, with an expansion or a contraction of one repeat unit. However, no association with changes in AR function was indicated because they had not been refractory to hormone therapy. Since these latter three tumors were associated with microsatellite instability, the changes might have been the result of an impairment of mismatch repair. This study indicates that AR gene mutations play a role, in only a subset of prostate cancer patients, in a treatment-refractory state.