Cigarette smoking is the most important modifiable risk factor for atherosclerosis. Endothelial dysfunction is an early event in atherogenesis, and we hypothesized that smoking might be associated with endothelial damage in the systemic arteries of otherwise healthy young adults. We studied noninvasively the brachial arteries of 200 subjects aged 15 to 57 years, all normotensive, nondiabetic with cholesterol level < or = 240 mg/dL and no family history of premature vascular disease: 80 control subjects aged 16 to 56 years (mean, 35), 80 current smokers aged 15 to 55 years (mean, 33), and 40 former smokers aged 25 to 57 years (mean, 38). Total lifetime amount smoked varied from 1 to 75 pack years in the smokers. Using high-resolution ultrasound, vessel diameter was measured at rest, during reactive hyperemia (with flow increase causing endothelium-dependent dilation), and after sublingual glyceryl trinitrate (GTN, an endothelium-independent vasodilator). Flow-mediated dilation (FMD) was observed in all the control subjects (10 +/- 3.3%; range, 4% to 22%) but was impaired or absent in the smokers (4 +/- 3.9%; range, 0% to 17%; P < .0001). FMD in the smokers was inversely related to lifetime dose smoked (6.6 +/- 4.0% in very light smokers, 4.0 +/- 3.1% in light smokers, 3.2 +/- 3.2% in moderate smokers, and 2.6 +/- 1.2% in heavy smokers; P < .01). FMD for the former smokers was 5.1 +/- 4.1% (range, 0% to 15%). In a multivariate model adjusting for age, sex, cholesterol, smoking history, and vessel size, former smoking was associated with a higher FMD than current smoking (P = .07); when only male former and current smokers were considered, the higher FMD was significant (P = .0001) but not for female smokers (P = .24). GTN caused dilation in all subjects (control subjects, 20 +/- 5.2%; smokers, 17 +/- 5.8%; former smokers, 17.4 +/- 5.4%). Vessel diameter, baseline flow, and degree of reactive hyperemia (Doppler estimated) were similar in all groups. Cigarette smoking is associated with dose-related and potentially reversible impairment of endothelium-dependent arterial dilation in asymptomatic young adults, consistent with endothelial dysfunction.