We confirmed a mutation of the androgen receptor gene as the cause for Kennedy's disease, also called “X-linked recessive spinal and bulbar muscular atrophy” or “bulbospinal neuronopathy.” The mutation is characterized by an increased size of a polymorphic tandem CAG repeat within the first exon of the gene. The study population consisted of 17 patients from seven families (five distinct kinships and two isolated cases). Two patients were as yet asymptomatic and had normal examinations. Four carrier females showed the mutant as well as the normal allele; none showed clinical features of Kennedy's disease. There was no large expansion of the mutation observed in three generations of one family. Phenotypic expression between and within families was variable and not related to the size of the mutation. This contrasts with the gene mutations found in myotonic dystrophy and fragile X syndrome, where increased severity of disease correlates with the number of tandem triplet repeats. The findings reported here appear to explain the failure to find genetic anticipation in Kennedy's disease. The DNA test for Kennedy's disease can now be used for definitive diagnosis and carrier detection. In addition, mutation analysis allows early detection, which has implications for potential treatment.