Clinical phenotypes and the CAG repeat size of the androgen receptor gene were assessed in 95 Japanese patients with X-linked recessive bulbospinal neuronopathy. There was an age- and duration-dependent deterioration of muscle strength, ADL scores, and plasma creatine kinase levels. However, there was no correlation between the presence of absence of gynecomastia or diabetes mellitus and the age at onset or duration of illness. Correlations were present between the CAG repeat size and the age at onset (P < 0.0001) as well as the presence or absence of gynecomastea (P < 0.05). Muscular weakness and ADL scores were also correlated with the CAG repeat size only when they were adjusted by the age at examination not by the duration of illness. These findings suggest that CAG repeat size is one of the determinant factors of disease progression. However, extensive variation in phenotypic severity in patients with the same size of CAG repeat was present even among the siblings, suggesting that other factors than CAG repeat size influence the phenotypic manifestation. The average gain of CAG repeat size expansion was a 1.4 repeat in paternal transmission which was more unstable than that in maternal transmission, but the magnitude of the expansion in paternal transmission was much smaller than is presently known for other diseases in which CAG repeat expansion is the responsible gene defect.