We determined inter-and intra-observer variability of measurements of QT and heart rate corrected QT (QTc) interval in newborn infants. In 55 newborns, a standard 12-lead electrocardiogram (ECG) with rhythm strip was analysed independently by three observers. QT and the preceding RR interval were measured in three different beats. The QTc interval was calculated using the Bazett formula (QTc = QT/ÖRR). Observers repeated their measurements after the recruitment period blinded for their first reading. Inter-and intra-observer variability were determined using the Bland and Altman method. In total, 59 ECGs were recorded. Due to artifacts, six had to be excluded. A marked inter-observer variability was observed in the measurement of the QT and QTc interval. Important intra-observer variability was denoted, with differences up to 80 ms for QTc. Conclusion: Mainly due to high heart rates, important inter-and intra-observer variability hampers a correct interpretation of the QT interval in newborn infants. We suggest not to rely on one single measurement and to apply different methods of measurement and a margin of safety when interpreting QT and heart rate corrected QT intervals in these age groups for making therapeutic decisions.