To study genotoxic effects of exposure to low levels of benzene, single-strand breaks (SSB) in DNA of leukocytes and urinary levels of the oxidative DNA adduct 8-hydroxydeoxyguanosine (8OHdG) were determined in 33 men occupationally exposed to benzene from gasoline and in 33 controls. The average exposure to benzene over a shift was determined by personal air sampling in the breathing zone. The 8-hr time-weighted average exposure to benzene was 0.13 ppm (mean value, range 0.003-0.6 ppm). Exposed workers had a significant increase of SSB (p = 0.04) over the shift compared with controls. Storage time of the samples seemed to affect the results. An analysis of samples with the same storage time showed a nonsignificant increase among the workers compared with controls. Urinary 8OHdG increased over the shift among the exposed workers but not among the controls. The highest values among the exposed workers were seen in late evening, with a slight decrease the next morning. Multiple linear analysis adjusting for smoking habits showed a significant association between the exposure level of benzene during the shift and the increase of 8OHdG in the urine over the shift among exposed workers (p = 0.02). These findings indicate a genotoxic effect in humans of benzene at relatively low exposure levels, that is, about 0.1 ppm (0.3 mg/m3).