Electrical dyssynchrony is postulated to be one of the main factors contributing to non-response of patients to cardiac resynchronization therapy (CRT). We applied inverse epicardial imaging computed from patient-specific geometry and body-surface potential recordings to assess global and regional electrical dyssynchrony. Patients were imaged pre- and post-device implantation, without and with pacing function (P-OFF and P-ON). The reconstructed maps of activation in the dyssynchronous pre-CRT rhythm agree with published contact mapping activation maps with earliest activation starting on the RV free wall and slowly spreading to the LV in a U-shaped pattern. The new ΔQRSi metric captures global pre-CRT dyssynchrony showing negative values (-0.32±0.10) with minimal variability beats (coefficient of variation 10±3.6%) while post-CRT pacing indicates positive values (0.18±0.11). The imaging method is well-suited to study electrical dyssynchrony and potentially guide CRT lead placement.