['To evaluate the impact of systemic ventricular (SV) morphology on mechano-electrical interaction (MEI) and compare the results with clinical profiles in Fontan patients.', 'Abnormal ventricular MEI causes systolic dysfunction of the right and left ventricles (RV, LV).', 'We evaluated serial changes (up to 15 years post-Fontan operation) in QRS duration (QRS-d, ms), catheterization-based hemodynamics and SV performance in 77 patients and compared them with 36 referents.', 'Preoperative QRS-d was wider in the Fontan patients (86+/-14 vs. 64+/-9, p<0.001) and steadily increased for the next 15 years (107+/-25 vs. 85+/-9, p<0.001). QRS-d was wider in non-LV type SV patients than those with LV morphology (p<0.05) with a positive association with age at repair (p<0.001). SV contractility (Ees) improved in the RV and LV groups (p<0.05), during the next 10-year Ees, ventriculoarterial coupling (Ea/Ees) and ventricular efficiency (SW/PVA) were superior in the LV group. QRS-d and its long-term change correlated with the corresponding SV volume and change (p<0.01), respectively, while the immediate postoperative volume reduction did not result in the QRS-d shortening. QRS-d correlated with Ees, Ea/Ees, SW/PVA, and peak oxygen uptake (VO(2)) (p<0.001) and QRS-d was one of the main determinants of peak VO(2) (r=-0.48, p<0.001).'] Does systemic ventricular morphology-associated increased QRS duration compromise the ventricular mechano-electrical and energetic properties long-term after the Fontan operation?