A marker for biological response to bladder carcinogen exposure was evaluated in a cross-sectional study of 504 workers at high risk due to a range of exposures to various carcinogenic aromatic amines, primarily 2-naphthylamine. A quantitative fluorescence cytology method using the DNA-binding dye, acridine orange, was employed to measure DNA in exfoliated urothelial cells. DNA hyperploidy (greater than 5 C) was observed in 16 (21.6%) of 74 workers who had been exposed compared with 15 (3.5%) of 430 workers who had not (p less than 0.001). The prevalence of DNA hyperploidy increased in a dose-response manner from 3.5% to 60% with increasing duration of exposure. The association between DNA hyperploidy and exposure persisted when adjustment was made for age and cigarette smoking (p = 0.0001). The prevalence of the marker was greatest for exposed workers who smoked (23%), and lowest for those who had no exposure and who had not smoked (2%). This study indicates that DNA hyperploidy can serve as a marker for identifying workers who are at increased risk in occupational groups exposed to bladder carcinogens.