QT-interval dispersion (QT dispersion) is a marker of the inhomogeneity of ventricular repolarisation. In human beings an increase in QT dispersion has been linked to sudden death and arrhythmias in several cardiac diseases. In dogs it has yet to be evaluated, and this study aimed to establish a normal reference range for QT dispersion and to assess the effects of cardiac disease upon it. Ten-lead ECGS were recorded from 81 dogs at two hospitals; satisfactory traces were obtained from 68 dogs, 32 of them normal and 36 with cardiac disease. The mean (sd) QT dispersion was 20.8 (18.2 ms) and the mean heart rate-corrected QT dispersion was 26.1 (23.6 ms). There was no significant effect of cardiac disease. The accuracy of the results may be questioned owing to the identification of several potential errors in the measurements.