Parental exposure to ionizing radiation increases the frequency of germline mutations detectable in the next generation1. Parental exposure can also increase the rate of mutation in somatic cells2,3 and confer a predisposition to cancer4,5,6 in offspring, suggesting that there could be an indirect effect of radiation on somatic genome stability that is transmissible through the germ line of the irradiated parents. We have found that this indirect effect extends to the germ line of unexposed first-generation offspring in mice, as revealed by an increased instability of repeat-DNA sequences in their descendants.