Although the effect of smoking on human health has been established as a major risk factor, the effect of passive smoking is controversial. The purpose of this study was to investigate the association between passive smoking and the risk of acute coronary syndromes (ACS) among nonsmokers. Eight hundred and forty-eight patients with the first event of ACS and 1078 cardiovascular disease-free matched controls completed a detailed questionnaire regarding their exposure to environmental smoke. Two hundred and ninety-seven (35%) of the patients and 259 (24%) of the controls were defined as nonsmokers and passive smokers, respectively. After controlling for several potential confounders, the results showed that nonsmokers exposed to cigarette smoke increased the risk of ACS by 51% (odds ratio = 1.51, 95% confidence interval 1.21-2.99) compared with nonsmokers not exposed to smoke. It was estimated that 34 coronary events per 134 subjects would occur as a result of passive smoking during their lifetime. Consequently, this study supports the hypothesis that passive smoking increases the risk of developing acute coronary syndromes. Given the high prevalence of cigarette smoking in many developing societies, the public health consequences of passive smoking with regard to coronary heart disease may be important.