PURPOSE To investigate the dynamics of chromosome aberrations in the blood cells of three workers severely exposed to neutrons and gamma-rays in a criticality accident that occurred in Tokai-mura, Japan, in 1999. MATERIALS AND METHODS The change with time of the frequency of' chromosome aberrations in the three patients was examined using a new analysis to score drug-induced prematurely condensed ring chromosomes (PCC-R) and a conventional meta-phase analysis. RESULTS The frequencies and cellular distributions of PCC-R, dicentrics and rings did not change significantly among the samples obtained at 9-48h after the accident while the first depletion of lymphocytes occurred. The distributions of these aberrations in the cells of two patients showed a slight overdispersion compared with a Poisson distribution reflecting neutron and non-uniform exposures. The dose response curve of rings paralleled that of dicentrics, but not PCC-R. The half-lives of PCC-R (8.5 months) and of rings (8.7 months) were shorter than that of dicentrics (13.5 months). CONCLUSIONS In the three patients of the Tokai-mura accident, lymphocytes in the circulating and extravascular pools had reached equilibrium at 9h, and highly damaged lymphocytes did not selectively move away from the circulatory system during the first rapid depletion of lymphocytes after exposure. Data on the in vivo half-life of PCC-R as well as dicentrics and rings obtained in the present study may be useful for retrospective dosimetry.