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Context: Recent studies have shown that passive smoking is a risk factor for ischemic heart disease and may be associated with vascular endothelial dysfunction. The acute effects of passive smoking on coronary circulation in nonsmokers are not known., Objective: To determine the acute effects of passive smoking on coronary circulation using coronary flow velocity reserve (CFVR), assessed by noninvasive transthoracic Doppler echocardiography., Design, Setting, and Participants: Cross-sectional study conducted from September 2000 to November 2000 among 30 Japanese men (mean age, 27 years; 15 healthy nonsmokers and 15 asymptomatic active smokers) without history of hypertension, diabetes mellitus, or hyperlipidemia., Main Outcome Measures: Coronary flow velocity reserve, calculated as the ratio of hyperemic to basal coronary flow velocity induced by intravenous infusion of adenosine triphosphate and measured in each participant before and after a 30-minute exposure to environmental tobacco smoke., Results: Heart rate and blood pressure responses to adenosine triphosphate infusion were not affected by passive smoking exposure in either group. Passive smoking exposure had no effect on basal coronary flow velocity in either group. Mean (SD) CFVR in nonsmokers was significantly higher than that in active smokers before passive smoking exposure (4.4 [0.91] vs 3.6 [0.88], respectively; P =.02), while CFVR after passive smoking exposure did not differ between groups (P =.83). Passive smoking exposure significantly reduced mean (SD) CFVR in nonsmokers (4.4 [0.91] vs 3.4 [0.73], respectively; P<.001)., Conclusions: Passive smoking substantially reduced CFVR in healthy nonsmokers. This finding provides direct evidence that passive smoking may cause endothelial dysfunction of the coronary circulation in nonsmokers.