Background: Cigarette smoking has several known detrimental effect on cardiovascular system. QT dispersion (QTd) is among the important predictors of cardiac death. Objective: To assess the acute effect of smoking of a single standard cigarette on QT and QTc (corrected QT) dispersion, hence on the risk of ventricular arrhythmias in healthy smokers and non-smokers. Methods: 111 healthy men with a mean±SD age of 32.5±10.2 years and with normal ECGs were included in this study. Two standard 12-lead ECGs were recorded for each subject; one baseline at least one hour after last smoking and the other, 7-10 minutes after smoking of a single standard filtered cigarette. The mean QT and QTc dispersion before and after smoking in different age groups in nonsmoker and smokers with different backgrounds of smoking habits were compared. Results: In the baseline ECG, the mean±SD QTd and the mean±SD QTc dispersion were 47±15, and 53±16 ms, respectively. These values increased to 57±16, and 60±16 ms, respectively in the second ECG (p<0.001). A significant rise in QTd was consistent in all age groups of non-smokers and smokers with different smoking backgrounds. Conclusions: Smoking of even a single cigarette in both smokers and non-smokers can lead to a rise in QTd.