This paper describes the epidemiological evidence on lung cancer and childhood leukemia in relation to traffic-related air pollution, with particular reference to diesel exhausts, polycyclic aromatic hydrocarbons (PAH) and benzene. Recent epidemiological studies strengthen the hypothesis of an increased lung cancer risk related to residential exposure to air pollution and to occupational exposure to diesel exhausts. The evidence on the carcinogenicity of several PAH mixtures comes from occupational studies, while the risk incurred by the general population is difficult to estimate. A few papers suggest that traffic-related air pollution may be associated with an increased risk of childhood leukemia. The observed relative risks are small but the exposure is widespread. Therefore, the overall impact of exposure to current levels of urban air pollution may be substantial.