A dysfunctional androgen receptor is able to cause variable phenotypes of androgen insensitivity or androgenicity in humans. In addition, also a polymorphism, the CAG repeat polymorphism in exon 1 of the androgen receptor gene (CAG)n, modulates androgen effects: androgen-induced target activities are attenuated corresponding to the length of triplet residues. Clinically, the (CAG)n polymorphism causes marked modulations of androgenicity in eugonadal men in various tissues and psychological traits and may cause the clinical picture of hypogonadism in the presence of normal testosterone concentrations. Also pharmacogenetic implications might exist in this regard: there appears to be a significant role of testosterone treatment of hypogonadal men as treatment effects have been demonstrated to be modulated by the number of (CAG)n in retrospective approaches.