A method and apparatus for measuring the QT interval of an electrocardiogram (ECG) signal is provided wherein the end of the T wave is identified from ECG data. The end of the T wave is defined as the first time of intersection at which an upright T wave of a first set of derived ECG signal data intersects an inverted T wave of a second set of derived ECG signal data. The intersection of the two sets of ECG data is along an isoelectric line within the trough after the positive T wave peak when the superimposed isoelectric baselines from the upright and inverted ECG signals demonstrate the best least squares fit.