Nucleotide repeat instability is associated with an increasing number of cancers and neurological disorders. The mechanisms that govern repeat instability in these biological disorders are not well understood. To examine genetic aspects of repeat instability we have introduced an expanded CAG trinucleotide repeat into transgenic mice. We have detected intergenerational CAG repeat instability in transgenic mice only when the transgene was maternally transmitted. These intergenerational instabilities increased in frequency and magnitude as the transgenic mother aged. Furthermore, triplet repeat variations were detected in unfertilized oocytes and were comparable with those in the offspring. These data show that maternal repeat instability in the transgenic mice occurs after meiotic DNA replication and prior to oocyte fertilization. Thus, these findings demonstrate that advanced maternal age is an important factor for instability of nucleotide repeats in mammalian DNA.