Wilson et al.,1934 argued that modifying the placement of ECG electrodes on the limbs should have no effect on ECG wave amplitudes unless they are moved onto the torso. Despite this, limb electrodes are routinely placed on the torso during exercise stress testing or when it is inconvenient or impossible to position them on the extremities. Under these conditions, monitoring the ECG, and in particular the ST segment is of great importance when diagnosing suspected myocardial ischemia/infarction. There remains no agreement on exactly how such electrode modification affects the 12 lead ECG (Mason & Likar, 1966; Takuma et al., 1995). We have therefore made 12 lead ECG measurements in 18 healthy subjects using both standard and modified electrode placements with the arm electrodes placed on the “anterior acromial region” and the leg electrodes on the “anterior superior iliac spine” (Takuma et al., 1995). Data was collected with a customised data collection system that averages ~120 heart beats per subject and performs automated offline analysis of the ECG waveform. Mean ± sem wave amplitudes were compared using a two tailed paired T-test with the Bonferroni correction for multiple comparisons. As expected, mean R and T heights in the precordial leads were not significantly different using the modified electrode placement (all < 0.01mV). R height and T height changed significantly by at least 0.06 ± 0.01mV in all the limb leads (except for lead aVR). In a clinical setting, the amplitude changes in leads III and aVF could be considered as potentially misleading (i.e., ≥ 0.1mV). Modification produced a rightward shift of ~15° in both QRS and T axes, but this would only be clinically important in patients with a borderline axis in the standard ECG. In no leads did clinically important ST depression or elevation occur (i.e., none had elevated or depressed J / ST(80) points by ≥ 0.1mV) and the largest change in ST height was only 0.03 ± 0.03mV (V3). Moving the limb electrodes of a 12 lead ECG to the modified positions for exercise stress testing and emergency monitoring (when the limbs are inaccessible) does produce measurable and reproducible changes in ECG wave amplitudes in healthy subjects. It does not however, produce a misleading ST segment change. Modified electrode placement therefore appears acceptable for ST segment monitoring under these commonly used conditions.