In this study we have compared the results of a method for the detection of cyclophosphamide in urine and the results of analysis of chromosomal aberrations in peripheral blood lymphocytes of four groups of subjects with various exposure statuses. These groups are 17 Dutch and 11 Czech exposed workers (mainly hospital nurses and pharmacy technicians) handling antineoplastic agents and 35 Dutch and 23 Czech controls (nurses, medical doctors, pharmacy and lab technicians) not handling these drugs. The groups were subdivided into smokers and non-smokers because of a confounding effect of smoking. Within the Dutch groups, the percentage of aberrant cells and the number of breaks per cell were increased for smokers compared to non-smokers. The percentage of aberrant cells was increased in Dutch exposed workers in comparison with Dutch control workers. Within the Czech groups the percentage of aberrant cells and the number of breaks per cell were increased in exposed workers in comparison with control workers. However, both Dutch and Czech smokers mainly contributed to the increase. The results suggest an additive effect of exposure and smoking in the Dutch subjects and a more than additive effect in the Czech subjects. In urine samples of three out of 11 Dutch exposed workers cyclophosphamide was found in a range of 0.1-0.5 micrograms/24 h. Higher levels were detected in the urine of eight out of 11 Czech exposed workers, a range of 0.1-2.9 micrograms/24 h. No correlation was observed between the amounts of cyclophosphamide excreted in urine on the one hand and the percentage of aberrant cells and the number of breaks per cell on the other hand. The present study is the first study showing that hospital workers having an increase in chromosome aberrations related to their work are exposed to at least one antineoplastic agent.