Cytogenetic analysis was performed in blood lymphocytes of 60 radiation workers employed in the Ignalina Nuclear Power Plant. The control group comprised 64 healthy male donors. Chromosome aberration analyses revealed no significant differences between the workers with an external gamma radiation exposure and the controls (1.60 vs. 1.65 CA/100 cells, P=0.85). An increase in the chromosome aberration frequency was observed for the workers with an additional internal and internal plus neutron exposure with significant differences for the total chromosome aberration, chromosome-type aberration and dicentric frequencies. The confounding effects of the internal and neutron exposure on the frequency of chromosome aberrations were determined. The impact of other factors (smoking, age, duration of employment, cumulative dose) was found to be non-significant.