Effect of long-term exposure to organic solvents on urinary excretion of 8-hydroxydeoxyguanosine (8-OHdG) in Japanese workers. Urinary excretion of 8-hydroxydeoxyguanosine (8-OHdG), a biomarker of oxidative stress, has been correlated with occupational exposure to organic solvents. We investigated whether the urinary excretion of 8-OHdG is associated with exposure to organic solvents for evaluating biological effects in humans. We conducted a cross-sectional study of 1,828 employees from 40 small- and medium-sized enterprises. Urinary levels of 8-OHdG were measured by high-performance liquid chromatography (HPLC). Urinary 8-OHdG levels were significantly and linearly associated with the logarithm of the duration of exposure to organic solvents in each job category after adjustment for age, gender, current smoking, current alcohol drinking, and creatinine clearance (p<0.05). The median urinary 8-OHdG levels were 0.14, 0.24, 0.21, 0.31, 0.37, and 0.53 (ng/mg creatinine) among workers with no exposure, short-term exposure, medium-term exposure, long-term exposure for more than 10 years, medium-term exposure for less than 10 years and long-term exposure for less than 10 years, respectively. The ratio of urinary 8-OHdG to creatinine was also significantly and linearly related to the logarithm of the duration of exposure (p<0.05). To investigate the possibility of biological effects of low-dose exposure to organic solvents, we assessed the effects of exposure duration on the adjusted levels of urinary 8-OHdG after subtracting the background of healthy employees not exposed to organic solvents. The adjusted urinary 8-OHdG levels were significantly and linearly associated with the logarithm of the duration of exposure (p<0.001). The results of the present study indicated that urinary 8-OHdG is a suitable biomarker for evaluating the biological effects of low-dose exposure to organic solvents. The results also indicated that workers with a history of exposure to organic solvents should be surveyed.