Recently published discussions of the possible role of chromosome aberrations in leukemogenesis (1) and aging (2) suggest that an increase in the frequency of such aberrations may carry with it an increased risk of illness. It is of more than academic interest, therefore, that some radiation workers do show a higher frequency of chromosome aberrations than is found in a comparable group taken from the population at large (3). In the present report we present data on chromosome aberrations in thirty-six radiation workers, thirty-three of whom have lifetime exposure doses in excess of 10 r. The frequency of chromosome aberrations in this group is significantly higher than in a control group despite the fact that the radiation was accumulated over many years at a median dose rate only about ten times the natural background radiation.