Expansion of the trinucleotide repeat (CAG)_n in the first exon of the androgen receptor gene is associated with a rare motor neuron disorder, X–linked spinal and bulbar muscular atrophy. We have found that expanded (CAG)_n alleles undergo alteration in length when transmitted from parent to offspring. Of 45 meioses examined, 12 (27%) demonstrated a change in CAG repeat number. Both expansions and contractions were observed, although their magnitude was small. There was a greater rate of instability in male meiosis than in female meiosis. We also found evidence for a correlation between disease severity and CAG repeat length, but other factors seem to contribute to the phenotypic variability in this disorder.