Context.-Cigarette smoking is a powerful risk factor for incident heart disease and stroke, but the relationship of active and passive smoking with the progression of atherosclerosis has not been described. Objective.-To examine the impact of active smoking and exposure to environmental tobacco smoke (ETS) on the progression of atherosclerosis. Design.-A longitudinal assessment of the relationship between smoking exposure evaluated at the initial visit and the 3-year change in atherosclerosis. Setting.-A population-based cohort of middle-aged adults from 4 communities in the United States. Participants.-A total of 10914 participants from the Atherosclerosis Risk in Communities (ARIC) study enrolled between 1987 and 1989. Main Outcome Measure.-Change in atherosclerosis from baseline to the 3- year follow-up as indexed by intimal-medial thickness of the carotid artery assessed by ultrasound and adjusted for demographic characteristics, cardiovascular risk factors, and lifestyle variables. Results.-Exposure to cigarette smoke was associated with progression of atherosclerosis. Relative to never smokers and after adjustment for demographic characteristics, cardiovascular risk factors, and lifestyle variables, current cigarette smoking was associated with a 50% increase in the progression of atherosclerosis (mean progression rate over 3 years, 43.0 μm for current and 28.7 μm for never smokers, regardless of ETS exposure), and past smoking was associated with a 25% increase (mean progression rate over 3 years, 35.8 μm for past smokers and 28.7 μm for never smokers). Relative to those not exposed to ETS, exposure to ETS was associated with a 20% increase (35.2 μm for those exposed to ETS vs 29.3 μm for those not exposed). The impact of smoking on atherosclerosis progression was greater for subjects with diabetes and hypertension. Although more pack-years of exposure was independently associated with faster progression (P<.001), after controlling for the number of pack-years, the progression rates of current and past smokers did not differ (P = .11). Conclusions.-Both active smoking and ETS exposure are associated with the progression of an index of atherosclerosis. Smoking is of particular concern for patients with diabetes and hypertension. The fact that pack-years of smoking but not current vs past smoking was associated with progression of atherosclerosis suggests that some adverse effects of smoking may be cumulative and irreversible.