The genetic risk run by workers occupationally exposed to styrene vapors was assessed in two different plants A and B, using the cytogenetic analysis of peripheral blood lymphocytes. In plant A engaged in the manufacture of polystyrene vessels the mean styrene exposure level was found to range between 70 and 150 mg . m-3, in plant B manufacturing sports boats, plastic slides for children and plastic guard-stones it reached the level of about 200 mg . m-3. The rate of aberrant cells (AB.C.) found in plant A workers (N = 36) at the time of first sampling was 1.38% and the value of break/cell (B/C) ratio was 0.015; at the second sampling the rate of AB.C. was 1.41% and the B/C ratio was 0.014. The group of matched controls (N = 19) was found to have 1.26% of AB.C. and 0.014 breaks per cell. Plant B workers (N = 22) exhibited at the first sampling 1.72% of AB.C. and their value of B/C ratio was 0.018, the group of matched controls (N = 22) had 1.36% of AB.C and the B/C ratio 0.015; the respective values at the time of second sampling were 2.81% for AB.C. rate and 0.029 for B/C ratio in the exposed and 1.89% for AB.C. rate and 0.021 for B/C ratio in the control group. It is concluded that styrene exposure levels below 100 mg . m-3 do not pose any serious genetic risk for the exposed population groups. The variations found in the degree of chromosome injury by smoking habits, drug intake pattern, or sex were not statistically significant.