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We investigated the influence of mild intraoperative hypothermia on the QTc-interval in 50 patients undergoing major cardiac surgery. To our knowledge no study has investigated the influence of mild intraoperative hypothermia on the QTc-interval in a prospective study design, yet. We assessed the QT-intervals in ECG prints, whenever the body temperature dropped by 0.1°C. For heart rate correction of the QT-intervals we used three popular formulas (Bazett, Fridericia and Framingham). From the original dataset we selected the temperature values and the corresponding QTc-intervals which were overlapping in all patients. In this way, we obtained a modified dataset with equalised values. We found a QTc-interval prolongation of 61.81 ± 49.09ms (Framingham), 85.05 ± 70.90ms (Fridericia) and 97.43 ± 85.91ms (Bazett) in the original dataset. In the modified dataset we found a QTc-interval prolongation of 15.23 ± 34.31ms (Framingham), 14.77 ± 59.96ms (Fridericia) and 16.95 ± 75.54ms (Bazett). P-values were p < 0.001 for all formulas in both datasets. Our results demonstrate that mild intraoperative hypothermia causes a statistically significant prolongation of the QTc-interval. We compared the results with the results of pharmacological studies. The ICH E14 guideline determines a QTc-interval prolongation of 5ms as risk factor for developing torsade de pointes tachycardia in pharmacological studies. Pharmacological studies show QTc-interval prolongations of 20ms and more for perioperative administered drugs. Mild intraoperative hypothermia is therefore a more important risk factor for developing torsade de pointes tachycardia than drug induced QTc-interval prolongation.