An unresolved question in the study of the polyglutamine neurodegenerative disorders is the extent to which partial loss of normal function of the mutant protein contributes to the disease phenotype. To address this, we studied Kennedy disease, a degenerative disorder of lower motor neurons caused by a CAG/glutamine expansion in the androgen receptor (Ar) gene. Signs of partial androgen insensitivity, including testicular atrophy and decreased fertility, are common in affected males, although the underlying mechanisms are not well understood. Here, we describe a knock-in mouse model that reproduces the testicular atrophy, diminished fertility, and systemic signs of partial androgen insensitivity that occur in Kennedy disease patients. Using this model, we demonstrate that the testicular pathology in this disorder is distinct from that mediated by loss of AR function. Testes pathology in 113 CAG knock-in mice was characterized by morphological abnormalities of germ cell maturation, decreased solubility of the mutant AR protein, and alterations of the Sertoli cell cytoskeleton, changes that are distinct from those produced by AR loss-of-function mutation in testicular feminization mutant mice. Our data demonstrate that toxic effects of the mutant protein mediate aspects of the Kennedy disease phenotype previously attributed to a loss of AR function.