The introduction of fluorescence in situ hybridisation techniques to cytogenetic biodosimetry has enabled the detection of stable aberrations, e.g. translocations, which can provide an integrated measure of radiation exposure. The opportunity to study stable aberration frequencies in lymphocyte cultures from occupationally exposed workers with well-documented dosimetry records enables the establishment of dose-response relationships which can be compared with those obtained from studies of populations with high acute exposures. Since there is good mechanistic evidence for the role of translocations in oncogenesis, this provides an insight into the risks associated with low-dose exposure for which no direct epidemiological evidence is available. This paper provides preliminary data on a group of 98 individuals with cumulative occupational doses > 500 mSv and 96 controls with doses < 50 mSv. An increased frequency of translocations was found for the high-dose group (20.50 +/- 0.78 x 10(-3) per genome equivalent) in comparison with the control group (11.71 +/- 0.59 x 10(-3) per genome equivalent). Further analysis of the control group revealed increasing frequencies of translocations with increasing age. An increase associated with ever-smoking status was found for the control group which appeared to be driven by the lower age groups but no overall effect of smoking was seen in the > 500 mSv group.