The concentrations of pollutants, especially polycyclic aromatic hydrocarbons (PAHs), originating from automobile emissions are high in areas around urban arterial roads. To investigate the possibility of using urinary 1-hydroxypyrene (1-POH) a metabolite of pyrene, as a marker for estimating the amount of human exposure to PAHs, both an animal experiment and an ecological correlation study were conducted. Rats were exposed to one of two sources of PAH: diesel engine emissions containing particulate matter and NO2 at average concentrations of 4.20 mg/m3 and 2.90 ppm, respectively, or, for the control group, air having the respective average concentrations of 0.01 mg/m3 and 0.02 ppm. The concentration of pyrene was 36 ng/mg in the particulate matter in the diluted diesel engine exhaust and 9.0 ng/g in the feed to the rats. Urinary 1-POH levels in the rats of the exposure group increased remarkably over those of the control group, 2.4 times as much by the 2nd week of exposure and 5.6 times by the 4th and 8th weeks. The ecological correlation study was conducted in 1988 and 1989 in two area of Tokyo along arterial roads (Meguro and Itabashi Wards) and in one suburban area (Higashiyamato City) to measure urinary 1-POH levels in elementary school children who lived in those areas. Urinary samples were collected in October in 1988, as well as in January, May, and July in 1989. Throughout the period of investigation, the schoolchildren in the highly NOx-polluted Meguro and Itabashi Wards showed significantly higher urinary 1-POH levels than the children in the less-polluted Higashiyamato City by a factor of 1.1-1.6. These results suggest that the urinary 1-POH level could be used as a good marker for estimating the amount of exposure of residents to PAHs.