Air samples were collected at the sides of busy traffic roads in downtown Kanazawa, Sapporo and Tokyo during the same periods both in the daytime and in the night, in winter and summer. We determined the levels of polycyclic aromatic hydrocarbons (PAHs) and nitropolycyclic aromatic hydrocarbons (NPAHs), which are known to be carcinogenic and/or mutagenic. PAHs and NPAHs were analyzed simultaneously with an HPLC system with fluorescence detection for PAHs and another HPLC system with chemiluminescence detection for NPAHs, respectively. The amounts of airborne particulates were larger in the daytime than in the night in all three cities. This result was thought to reflect the diurnal change of such urban activities as traffic volume. Many of the PAH and NPAH compounds showed higher concentrations in the daytime than in the night, and also showed higher concentrations in winter than in summer in the three cities. This result coincided with the trends observed in our continuous surveys in downtown Kanazawa for several years. However, 5-ring PAHs, such as benzo [a] pyrene, and dinitropyrenes in Tokyo did not show such diurnal or seasonal differences. The composition ratio of NPAHs and the contribution of diesel-engine vehicles to the atmospheric concentrations of NPAHs were calculated from all the data of this survey. The concentration ratio of dinitropyrenes to 1-nitropyrene became larger with an increase in the ratio of diesel-engine vehicles to the total vehicles registered, suggesting a large contribution of diesel-engine vehicles to the concentrations of these NPAHs in urban air.