BACKGROUND: There is limited evidence on whether particulate matter (PM) can augment the progression of atherosclerosis; furthermore, the specific attributes of PM responsible for health effects are unclear. We developed models to predict exposure to PM <10 μm (PM10) and also to predict a measure of oxidative potential (the capacity of particles to induce oxidative damage). Our objectives were (1) to estimate the association between PM10 and carotid intima-media thickness, a measure of subclinical atherosclerosis, and (2) to compare this association with that of PM10 weighted by its oxidative potential (PM10*OP). METHODS: Analysis was based on 2348 participants of the Whitehall II cohort of British civil servants who had intima-media thickness measured between 2003 and 2005 and lived in Greater London. Weekly PM10 and PM10*OP were predicted at each participant's residence. Primary exposure metrics were defined as PM10 and PM10*OP averaged over the year before scan. We estimated associations between exposure metrics and intima-media thickness using generalized linear regression models. RESULTS: An interquartile range increase (5.2 μgm(-3)) in PM10 was associated with a 5.0% (95% confidence interval = 1.9% to 8.3%) increase in intima-media thickness after covariate adjustment. The association for an interquartile range change in PM10*OP (1.5 m(-3)) was weaker: 1.2% (0.2% to 2.2%). CONCLUSIONS: These findings support a relationship between PM exposure and atherosclerosis. PM weighted by this particular measure of oxidative potential was not more predictive of the extent of atherosclerosis than PM mass concentration.