Background: Active cigarette smoking has been established as a potent risk factor for carotid atherosclerosis in clinical populations; however, neither the role of active smoking in general populations nor the impact of environmental tobacco smoke has been well described. Methods: The association between carotid artery wall thickness and cigarette smoking was studied in 12 953 black and white men and women, aged 45 to 65 years, examined in the Atherosclerosis Risk in Communities Study. Participants were classified as current smokers (n = 3525), past smokers (n = 4315), never smokers reporting weekly exposure to environmental tobacco smoke (ETS or 'passive smoking') of at least 1 hour (n = 3339), or never smokers reporting no weekly exposure to ETS (n = 1774). Carotid artery intimal-medial thickness (IMT) was measured by B-mode ultrasound. Results: Increased IMT was observed in each category, in order from smallest to greatest increase: never smokers not exposed to ETS, never smokers exposed to ETS, past smokers, and current smokers. The larger IMT observed in the nonsmoking group exposed to ETS compared with the nonsmokers not exposed persisted after control for diet, physical activity, body mass index, alcohol intake, education, and major cardiovascular risk factors. Among past and current smokers, increased pack-years of exposure was associated with increased IMT. Among nonsmoking men exposed to ETS, there was a significant increase in IMT with increasing number of hours per week of ETS exposure. Conclusions: These data confirm the strong relationship between active smoking and carotid artery IMT and provide initial evidence that passive smoking exposure is related to greater IMT. Increasing exposure to cigarette smoke (either pack-years of active smoking or hours of ETS) was significantly related to increased IMT.