Background. Active and passive smoking have been found to be associated with clinical atherosclerotic disease. To explore the effects of smoking on atherogenesis, we investigated the relationship of past and current active and passive smoking to carotid atherosclerosis in middle-aged adults. Methods. The study population consisted of 2,073 middle-aged residents of Washington County, Maryland. Information on active smoking and exposure to environmental tobacco smoke (ETS) was obtained from a 1975 census and from the baseline visit of the Atherosclerosis Risk in Communities (ARIC) Study in 1987-1989. Carotid artery intimal-medial wall thickness, measured by B-mode ultrasound methods in 1987-1989, was used as an indicator of carotid atherosclerosis. Mean intimal-medial wall thickness (IMT) was adjusted for age, gender, cardiovascular risk factors, and education using multiple linear regression. Results. The lowest mean IMT was found among never smokers who had never been exposed to ETS (mean ± standard error: 0.706 ± 0.013 mm). Exposure to ETS in one or both time periods was associated with increased IMT among never smokers (ETS in 1975 only: 0.731 ± 0.022; ETS in 1987-1989 only: 0.738 ± 0.011; ETS in both periods: 0.734 ± 0.012). Active smoking in 1975 was also associated with increased IMT. The greatest mean intimal-medial wall thickness was found among persons who were current smokers in both time periods (0.807 ± 0.009). Conclusions. Both past and current passive and active smoking are associated with increased carotid intimal-medial wall thickness. © 1995 by Academic Press, Inc.