Diesel vehicles are an important source of emission of air pollutants, particularly toxic compounds with potential health impacts. Current developments in engine design and fuel quality are expected to reduce these emissions, but many older vehicles will make a major contribution to urban pollutant concentrations and related health impacts for many years. In this study the relative inhalation risk of emissions of a range of toxic compounds are reported using data from a study of in-service vehicles driven through urban drive cycles using a range of diesel fuel formulations. The fuels ranged in sulfur content from 24 to 1700 ppm, and in total aromatics from 7.7 to 33 mass%. Effects of vehicle type and fuel composition on the risk of emissions of a range of toxic species are reported. The results show that the inhalation cancer risk is dominated for most of the vehicles and the testing modes by emissions of the combustion derived products, particularly benzene, naphthalene, and formaldehyde, and not by the polycyclic aromatic hydrocarbons such as benzo(a)pyrene. Based on the relative risk represented by these toxic species, improved fuel quality does not result in significant reductions in the relative inhalation cancer risk. However, this conclusion may be affected by additional toxic species and fine particles present in diesel exhaust, which were not included in this study.