Tobacco cigarette smoking is associated with increased sudden death risk, perhaps through adverse effects on ventricular repolarization. The effect of electronic (e-)cigarettes on ventricular repolarization is unknown. The objective of the study was to test the hypothesis that tobacco cigarettes and e-cigarettes have similar adverse effects on electrocardiogram (ECG) indexes of ventricular repolarization and these effects are attributable to nicotine. ECG recordings were obtained in 37 chronic tobacco cigarette smokers, 43 chronic e-cigarette users, and 65 nonusers. Primary outcomes, Tpeak to Tend (Tp-e), Tp-e/QT ratio, and Tp-e/QTc ratio, were measured in tobacco cigarette smokers pre-/post-straw control and smoking one tobacco cigarette and in e-cigarette users and nonusers pre-/post-straw control and using an e-cigarette with and without nicotine (different days). Mean values of the primary outcomes were not different among the three groups at baseline. In chronic tobacco cigarette smokers, all primary outcomes, including the Tp-e (12.9 ± 5.0% vs. 1.5 ± 5%, P = 0.017), Tp-e/QT (14.9 ± 5.0% vs. 0.7 ± 5.1%, P = 0.004), and Tp-e/QTc (11.9 ± 5.0% vs. 2.1 ± 5.1%, P = 0.036), were significantly increased pre-/post-smoking one tobacco cigarette compared with pre-/post-straw control. In chronic e-cigarette users, the Tp-e/QT (6.3 ± 1.9%, P = 0.046) was increased only pre/post using an e-cigarette with nicotine but not pre/post the other exposures. The changes relative to the changes after straw control were greater after smoking the tobacco cigarette compared with using the e-cigarette with nicotine for Tp-e (11.4 ± 4.4% vs. 1.1 ± 2.5%, P < 0.05) and Tp-e/QTc (9.8 ± 4.4% vs. -1.6 ± 2.6%, P = 0.05) but not Tp-e/QT(14.2 ± 4.5% vs. 4.2 ± 2.6%, P = 0.061) . Heart rate increased similarly after the tobacco cigarette and e-cigarette with nicotine. Baseline ECG indexes of ventricular repolarization were not different among chronic tobacco cigarette smokers, electronic cigarette users and nonusers. An adverse effect of acute tobacco cigarette smoking on ECG indexes of ventricular repolarization was confirmed. In chronic e-cigarette users, an adverse effect of using an e-cigarette with nicotine, but not without nicotine, on ECG indexes of ventricular repolarization was also observed.NEW & NOTEWORTHY Abnormal ventricular repolarization, as indicated by prolonged Tpeak-end (Tp-e), is associated with increased sudden death risk. Baseline ECG indexes of repolarization, Tp-e, Tp-e/QT, and Tp-e/QTc, were not different among tobacco cigarette (TC) smokers, electronic cigarette (EC) users, and nonsmokers at baseline, but when TC smokers smoked one TC, all parameters were prolonged. Using an electronic cigarette with nicotine, but not without nicotine, increased the Tp-e/QT. Smoking induces changes in ECG indexes of ventricular repolarization associated with increased sudden death risk.