The genetic mutation of X-linked recessive bulbospinal neuronopathy is amplification of a polymorphic tandem CAG repeat in the androgen receptor gene. We studied this CAG repeat in 26 Japanese patients from 21 families with X-linked recessive bulbospinal neuronopathy. The number of CAG repeats was significantly correlated with the age at onset of limb muscular weakness (r = -0.596, p < 0.001) and age-adjusted scored disability (r = 0.446, p < 0.03). The length of the CAG repeat therefore seems to be a determinant factor of clinical severity.