Smoking is linked to atherosclerosis and coronary heart disease (CHD) in older adults. However, evidence that smoking affects coronary atherosclerosis in young people is incomplete. The Pathobiological Determinants of Atherosclerosis in Youth (PDAY) Study collected arteries, blood, and other tissues from persons 15 to 34 years of age dying of external causes and autopsied in forensic laboratories. Lesions in the proximal left anterior descending coronary arteries (LAD) from 1127 subjects were graded microscopically according to the American Heart Association criteria. Among individuals with advanced lesions (Grade 4 or 5), smokers had a greater prevalence of Grade 5 lesions than non-smokers (odds ratio 9.61, 95% confidence interval 2.34-39.57), a difference suggesting that smoking accelerates the transition from Grade 4 to Grade 5 lesions. This association occurred among both men and women, and among persons with and without other CHD risk factors. The difference in qualities of advanced lesions suggests that smoking possibly accelerates the transition from Grade 4 to Grade 5 lesions by promoting thrombosis and accretion on the intimal surface of the plaque.