Background: Catheter ablation is a successful tool for the treatment of ventricular arrhythmias (VA) but procedures are long and complex. Pacemapping is used to locate the site of origin (SO) of VA but there is currently no guide as to where the catheter should be placed in order to locate the SO. Aims: 1. To identify variables related to intracardiac catheter location that may alter the paced QRS morphology. 2. To investigate the relationship between change in surface ECG morphology and distance within the ventricles. 3. To test a novel software algorithm that uses the above relationship m individual patients to prospectively locate the SO of VA. Methods: Patients undergoing ablation of VAs were enrolled. Data from pacemapping within tissue with preserved myocardial voltage was collected and measurements made on QRS morphology and intracardiac electro grams. Data on catheter position and chamber geometry were extracted from a 3D mapping system into custom software to construct linear regression models of distance against morphology difference. A novel software algorithm to automatically locate the SO of VAs was prospectively tested. Results: 935 pacemaps were collected in 68 patients over 74 procedures. QRS width was associated with pacing within dense scar tissue. 6219 pacemap pairs were used in distance-similarity regression models. Distance was significantly and positively associated with change in ECG morphology between patients and across ventricles despite the presence of structural heart disease. The software algorithm was tested on 46 clinical VAs in 35 separate procedures and correctly identified the exit site of 45/46 VAs. Conclusions: There is a robust relationship between distance and difference in surface EeG morphology when pacemapping in myocardium with preserved voltage. This relationship can be constructed in individual patients using a software algorithm and used to identify the SO of VAs in an automated manner.