Peripheral blood lymphocyte cultures of 116 smokers and 80 non-smokers who were occupationally exposed to uranyl compounds were analysed for sister-chromatid exchanges (SCEs). Blood samples were collected from 59 non-smokers (control group I) and 47 smokers (control group II) who were not exposed to uranium for control data. A significant increase in SCEs was observed among both smokers and non-smokers exposed to uranyl compounds when compared to their respective controls. In controls, a significant increase in the frequency of SCEs was observed in smokers when compared to non-smokers.