Background: Passive cigarette smoking is a known cause of a variety of diseases. However, most of the studies done in this field have focused on its chronic effects on human health. Studies considering the acute effect of smoking on QT dispersion, on the other hand, with its known predictive role in the occurrence ventricular arrhythmia and sudden cardiac death are sparse. Objectives: This study aims at determining possible relation between acute exposure to others’ cigarette smoke and QT dispersion in normal subjects. Materials and Methods: In this prospective study, 95 healthy male volunteers 16 to 62 years (meanًSD: 29.8±9.5) and normal baseline ECG were selected. Within 5 minutes of inhaling the smoke of one filtered cigarette burned at a distance of one meter a second ECG was obtained. We measured QT intervals in each of the 12 leads and corrected them according to the heart rate (QTc). Difference of maximum and minimum measured QT and QTc intervals amongst 12 leads (QT dispersion (QTd) and QTc dispersion (QTcd), respectively) were compared. Results: Mean±SD QTd were 50.8±17.8 ms before and 73.6±29.2 ms after passive smoking. Mean±SD QTcd were also increased from 62.2±20.9 ms to 85.2±30.8ms (p<0.001). Conclusion: Passive smoking may acutely affect myocardial vulnerability to arrhythmia by increasing QT dispersion. More strict rules against smoking in all public places are endorsed in order to decrease the incidence of this preventable cause of illness and death.