Abstract Objective Diesel exhaust particulate (DEP), a major component of urban air pollution, has been linked to atherogenesis and precipitation of myocardial infarction. We hypothesized that DEP exposure would increase and destabilise atherosclerotic lesions in apolipoprotein E deficient (ApoE−/−) mice. Methods ApoE−/− mice were fed a ‘Western diet’ (8 weeks) to induce ‘complex’ atherosclerotic plaques, with parallel experiments in normal chow fed wild-type mice. During the last 4 weeks of feeding, mice received twice weekly instillation (oropharyngeal aspiration) of 35 μL DEP (1 mg/mL, SRM-2975) or vehicle (saline). Atherosclerotic burden was assessed by en-face staining of the thoracic aorta and histological examination of the brachiocephalic artery. Results Brachiocephalic atherosclerotic plaques were larger in ApoE−/− mice treated with DEP (59±10%) than in controls (32±7%; P = 0.017). In addition, DEP-treated mice had more plaques per section of artery (2.4±0.2 vs 1.8±0.2; P = 0.048) and buried fibrous layers (1.2±0.2 vs 0.4±0.1; P = 0.028). These changes were associated with lung inflammation and increased antioxidant gene expression in the liver, but not with changes in endothelial function, plasma lipids or systemic inflammation. Conclusions Increased atherosclerosis is caused by the particulate component of diesel exhaust producing advanced plaques with a potentially more vulnerable phenotype. These results are consistent with the suggestion that removal of the particulate component would reduce the adverse cardiovascular effects of diesel exhaust.