Large scale cytogenetic studies of nuclear power plant workers and hospital workers occupationally exposed to X- and gamma -rays were performed using the micronucleus-centromere assay. Apart from the effect of the radiation burden, the influence of age, gender and smoking habits on the number of the centromere-positive, centromere-negative and on the total number of micronuclei per 1000 binucleated cells was investigated. Both studies showed that the systematic increase of the micronucleus frequency with age, 0.24-0.40 micronuclei per year, observed in the non-smoking populations, is mainly due to an increased chromosome loss, reflected by the centromere-positivity of the micronuclei. The study of the hospital workers, which consisted of females for two thirds, showed that the micronucleus frequencies are 30 % higher in females than in males, which can be attributed again to a higher chromosome loss, very probably of the X-chromosome. For a restricted number of workers, four hospital workers and one nuclear power plant worker, the micronucleus frequency was very high ( more than 50). The FLSH analysis showed that for these cases this high number was due to centromere-positive micronuclei. The analysis of the data for the influence of smoking habits on the micronucleus frequency was negative: no correlation between smoking and an enhanced number of centromere-positive or -negative micronuclei was observed. No statistically significant differences at the 95 % confidence level between the exposed and the control groups of both studied populations were observed even not after classification of the workers according to their radiation burden accumulated over the 10 years preceding the venepuncture On the other hand, a linear regression to the data of the radiation workers of the nuclear industry showed a slight increase of the total number of micronuclei and the number of centromere-negative micronuclei with dose ( 0.025 MN per mSv). Based on this result a statistically significant enhancement of the MN frequency may not be expected for workers with dose below 20 mSv per year, the ICRP 60 limit. The data bank, resulting from these studies comprising about 350 workers, will be of great value in the follow up of workers with high radiation burden as in the case of radiation accidents.