Increased length of a protein-coding CAG repeat within the androgen receptor gene appears to be the only type of mutation responsible for spino-bulbal muscular atrophy (SBMA or Kennedy disease). We have analysed a large 4-generation SBMA family and found that the mutant allele was unstable upon transmission from parent to child, with a documented variation from 46 to 53 repeats and a tendency to increase in size (7 increases and a single decrease in 17 events), which appeared stronger upon transmission from a male than from a female. Our results suggest also limited somatic instability of the abnormal allele, with observable variation of up to 2-3 repeats. This indicates that the behavior of the CAG repeat is similar to that observed for small premutations in the fragile X syndrome, or small abnormal alleles in myotonic dystrophy, two diseases which are caused by expansion of an unstable trinucleotide repeat.