Kennedy disease is a disorder with progressive motor neuron degeneration that is caused by trinucleotide repeat expansion in the androgen receptor gene. The disease mechanism likely involves toxicity of an expanded polyglutamine tract in the androgen receptor protein. This mechanism is probably shared by other neurodegenerative disorders with polyglutamine expansion, including Huntington disease. Attempts at reproducing the Kennedy disease phenotype by introducing the expanded androgen receptor into cultured neuronal cells and transgenic animals have thus far been unsuccessful, but recently developed model systems with other expanded polyglutamine constructs should allow the pathogenesis of these diseases to be elucidated.