Aging is thought to negatively affect multiple cellular processes including the ability to maintain chromosome stability. Chromosome instability (CIN) is a common property of cancer cells and may be a contributing factor to cellular transformation. The types of DNA aberrations that arise during aging before tumor development and that contribute to tumorigenesis are currently unclear. Mdm2, a key regulator of the p53 tumor suppressor and modulator of DNA break repair, is frequently overexpressed in malignancies and contributes to CIN. To determine the relationship between aging and CIN and the role of Mdm2, precancerous wild-type C57Bl/6 and littermate-matched Mdm2 transgenic mice at various ages were evaluated. Metaphase analyses of wild-type cells showed a direct correlation between age and increased chromosome and chromatid breaks, chromosome fusions and aneuploidy, but the frequency of polyploidy remained stable over time. Elevated levels of Mdm2 in precancerous mice increased both the numerical and the structural chromosomal abnormalities observed. Chromosome and chromatid breaks, chromosome fusions, aneuploidy and polyploidy were increased in older Mdm2 transgenic mice compared with wild-type littermates. Unexpectedly, chromosome fusions, aneuploidy and polyploidy rates in Mdm2 transgenic mice, but not chromosome and chromatid breaks, showed cooperation between Mdm2 overexpression and age. Notably, Mdm2 overexpression promoted gains in one or more chromosomes with age, while it did not affect the rate of chromosome loss. Therefore, aging increased specific forms of genomic instability, and elevated Mdm2 expression cooperated with aging to increase the likelihood of gaining certain chromosomal abnormalities of the kind thought to lead to cancer development.