Background Targeted Temperature Management (TTM) improves outcomes after cardiac arrest but may affect the QT and QTc intervals which could increase the chance of subsequent arrhythmia. We report here the effects of TTM on both computer-derived and manually calculated QT and QTc as well as the relationship of the length of the QTc and serious arrhythmia in a retrospective single-center experience. Methods 193 patients undergoing TTM for cardiac arrest were studied. 12-lead electrocardiograms (ECG) were measured before, during and after TTM. We assessed the QT and Bazett-corrected QT intervals (QTc) and examined the relationship between QTc and the occurrence of malignant arrhythmias. Results Both the QT and QTc increased during TTM whether derived manually or from the computer algorithm, although values were different with the two methods. Neither the QT nor the QTc were significantly longer in those patients with malignant arrhythmias. Conclusions QT and QTc prolong during TTM. There was no differential increase in the QTc in patients who experienced malignant arrhythmias. While the mechanism of QTc prolongation is not clear, it would not appear that the degree of QTc prolongation has an adverse effect on cardiac rhythm during TTM.