The X-linked androgen receptor (AR) gene contains two polymorphic trinucleotide repeat segments that code for polyglutamine and polyglycine tracts in the N-terminal trans-activation domain of the AR protein. Changes in the lengths of these polymorphic repeat segments have been associated with increased risk of prostate cancer, an androgen-dependent tumor. Expansion of the polyglutamine tract causes a rare neuromuscular disease, spinal bulbar muscular atrophy, that is associated with low virilization, reduced sperm production, testicular atrophy, and infertility. As spermatogenesis is exquisitely androgen dependent, it is plausible that changes in these two repeat segments could have a role in some cases of male infertility associated with impaired spermatogenesis. To test this hypothesis, we examined the lengths of the polyglutamine and polyglycine repeats in 153 patients with defective sperm production and compared them to 72 normal controls of proven fertility. There was no significant association between the polyglycine tract and infertility. However, patients with 28 or more glutamines (Gln) in their AR had more than 4-fold (95% confidence interval, 4.9-3.2) increased risk of impaired spermatogenesis, and the more severe the spermatogenic defect, the higher the proportion of patients with a longer Gln repeat. Concordantly, the risk of defective spermatogenesis was halved when the polyglutamine tract was short (< or = 23 Gln). Whole cell transfection experiments using AR constructs harboring 15, 20, and 31 Gln repeats and a luciferase reporter gene with an androgen response element promoter confirmed an inverse relationship between Gln number and trans-regulatory activity. Immunoblot analyses indicated that the reduced androgenicity of the AR was unlikely to be due to a change in AR protein content. The data indicate a direct relation between length of the AR polyglutamine tract and the risk of defective spermatogenesis that is attributable to the decreased functional competence of AR with longer glutamine tracts.