Left ventricular (LV) dysfunction is a predictor of adverse outcomes in patients with repaired tetralogy of Fallot (TOF). However, the mechanisms for LV dysfunction are not well understood. The aim of the study was to determine whether the prolonged QRS duration of right branch bundle block was associated with adverse LV mechanics. Seventy-five patients (mean age 31 +/- 2 years) with repaired TOF were studied. LV and right ventricular (RV) volumes and ejection fractions (EFs) were assessed using cardiac magnetic resonance imaging. Vector velocity imaging was used to assess longitudinal strain and intraventricular dyssynchrony. Prolonged QRS duration was associated with increased RV and LV dimensions (p = 0.01) and decreased function (RVEF r = -0.60, p <0.001 and LVEF r = -0.77, p <0.001). In addition, prolonged QRS duration was associated with heterogeneous ventricular mechanical activation and reduced strain in the lateral and septal left ventricle walls. Degree of intraventricular dyssynchrony correlated with LVEF (r = -0.59, p <0.001), QRS duration (r = 0.74, p <0.001), and septal strain (r = 0.70, p <0.001). In conclusion, LV dysfunction and dyssynchrony were observed in patients with TOF and were associated with QRS duration. It was possible that abnormal LV mechanics in combination with RV dysfunction may explain the relation between QRS duration and adverse cardiac outcomes.