Chromosome analyses were performed in blood lymphocytes of 25 subjects continuously living in houses with indoor radon (222Rn) concentrations exceeding 4-60-fold the German average of 50 Bqm-3. The mean frequency of cells containing dicentrics + ring chromosomes (1.3 +/- 0.3/1000 cells) and the incidence of dicentrics + ring chromosomes per cell (1.5 +/- 0.4 x 10(-3)) were significantly increased compared to the control levels (0.54 +/- 0.11 x 10(-3) for both endpoints). Taking into account the individual radiation history over the last 10 years prior to blood sampling and the life time of peripheral lymphocytes, weighted cumulative radon exposures at the time of blood sampling between 700 and 6300 Bqm-3a were derived. Although individual exposures could not be inferred from the aberration rates, a tendency for an exposure-effect relationship became apparent for two groups of subjects with a mean weighted cumulative radon exposure above and below 1800 Bqm-3a.