In these past decades an important increase in the prevalence of allergic respiratory diseases has been documented in most countries of the world with large differences being reported within different areas, particularly in industrialized countries. Persistent environmental exposure to particulate air pollution from motor vehicles has been suggested to be an important factor contributing to the observed increased prevalence of allergic diseases. Data from various investigators in different parts of the world have shown an important association between environmental levels of motor vehicle exhaust emissions and increased symptoms of asthma and rhinitis. In addition, recent human and animal laboratory-based studies have shown that particulate toxic pollutants, and especially diesel exhaust particles, can enhance allergic inflammation and induce the development of allergic immune responses. This article reviews the current state of knowledge on the role of diesel exhaust particles in the susceptibility to allergy. It scrutinizes the epidemiological evidence that supports the causative link between particulate air pollution from motor vehicles and the increasing prevalence in allergic conditions and the immunologic mechanisms by which diesel exhaust particles enhance the susceptibility to allergy.