Air pollution is associated with cardiovascular mortality. Inhaled ultrafine particles translocate into the blood. Amine-polystyrene ultrafine particles significantly enhance experimental thrombus formation in a damaged hamster vessel and shorten the closure time in the Platelet Function Analyser. Diesel exhaust particles are thrombogenic within one hour of intratracheal instillation and shorten the closure time ex vivo. These experimental observations provide a plausible biological explanation for the epidemiologically established link between air pollution and acute myocardial infarction.