Objective To investigate the correlation of CAGs repeat size and age of onset in patients with Kennedy's Disease (KD).Methods We detected the number of CAG repeats in the androgen receptor genes in 30 patients with KD.The correlation of CAGs repeat size with age of onset was analyzed.At the same time,the Appel scale that could represent the degree of motor functional impairment was scored in every patient.The correlation of Appel scale with CAGs repeat size and the course of disease were analyzed.Results Significant correlation was found between the number of CAGs with age of onset (r= -0.671,P <0.01 ).There was also correlation between the Appel score and the course of disease (r=0.855,P<0.01 ),but no correlation between the Appel score and the number of CAGs (r =0.100,P =0.601 ).Conclusions It is found that in Kennedy' disease,as well as in other CAG repeat diseases,the length of polyglutamine tract determines the age of onset ,but has no correlation with the severity of the disease.