The rate of genome damage elimination after therapeutic exposure to ionising radiation was estimated in stage I testicular seminoma patients monitored over a seven-year follow-up. DNA damage elimination in peripheral lymphocytes of ten subjects was analysed by the chromosome aberration assay. Seven years after the end of radiotherapy, significantly increased frequency of ring and dicentric chromosomes was still detected in comparison with baseline values. These results indicate the induction of genome instability. Long-term follow-up studies of cancer patients after radiotherapy could give us valuable information on the rate of genome damage elimination after exposure to ionising radiation and about the duration and manifestation of genome instability. This may be used in health risk assessment related to the possible development of secondary neoplasia. Studies such as this could have a great value both for oncology and radiation protection management protocols, especially after accidental overexposures.