The decrease of unstable chromosomal damages (dicentrics, rings and acentric fragments) was observed in circulating peripheral blood lymphocytes after exposure to ionizing radiation. The study comprised 100 subjects, all medical staff occupationally exposed to X-rays, who were re-examined for unstable chromosomal aberrations after 3 months (12 subjects), 6 months (31 subjects), 9 months (13 subjects), 12 months (21 subjects), over 24 months (14 subjects), and over 36 months (9 subjects) controls were 60 subjects who were not exposed to ionizing radiation or chemical mutagens at their workplaces. The results did not show time-dependent recovery from chromosomal aberrations in all cases.