Assessment of endocardial activation using filtered catheter-recorded electrograms has become a standard clinical technique. This study aims to assess factors affecting the accuracy of this process. The simulation study confirmed the ability of catheterbased signals to assess relative (“early vs. late”) timing of ventricular activation. However, absolute errors were high, amounting to 10% of the activation interval. Computer-based investigations provide insight into how relevant parameters (catheter size and angle, filtering) affect accuracy. This may allow for further improvement of catheter-based techniques.