The goal of this study was to assess the persistence of chromosomal aberrations and micronuclei of three victims 2 y after accidental radiation exposure to Co gamma rays. Traditional chromosome aberration analysis was performed by scoring the dicentric chromosomes (dic) and rings (r) in peripheral blood lymphocytes. Micronuclei were detected using the cytokinesis block micronucleus assay. G-banding and semi-automatic karyotype analysis was used to record translocations (t), inversions (inv) and deletions (del). The frequency of unstable chromosomal aberrations (dicentrics and rings) remained at high levels 6 mo after the accident. Two years after exposure, the frequency was reduced to 4-11% in the three victims. However, stable chromosome aberrations, which were detected by G-banding and included t, inv, and del, remained at a high level and have an obvious dose-dependent relationship even 2 y post-exposure. The frequency of micronuclei decreased faster than that of chromosome aberrations, reaching almost a normal level two years after the accident, especially for the child victim. Unstable chromosome aberrations reduced gradually, but the stable aberration remained at a high level along with the time-lapse. The micronucleus assay was less valuable for assessing long-term effects after high dose irradiation.