Introduction: Early repolarization syndrome (ERS) is a well-recognized ECG phenomenon, whose arrhythmogenic potential is still controversial. There is data suggesting that horizontal ST segment ERS in inferolateral leads, the so-called “malignant ERS”, is more frequently encountered in patients with sudden cardiac death. There are no parameters to differentiate “malignant” from “benign” ERS. Methods: We studied 67 subjects (23 with infero-lateral, horizontal ST elevation ERS- HST, 21 with infero-lateral, ascendant ST elevation ERS – AST, and 23 normals – C, with similar age and sex), by 2D and 3D echo, tissue Doppler, and speckle tracking imaging. We assessed structural and functional echo parameters, and cardiac synchrony (intra-, inter- and atrioventricular). Intraventricular synchrony was measured from standard deviation of time-to-peak systolic velocity (SD-Ts) and strain (SD-Sl), in the 12 basal and midventricular LV segments from the apical views. Results: Both ERS groups had higher LV end diastolic volume (HST-126 ml, AST-114 ml, C-97 ml, p-0.01) and mass (HST-94.2 g/m², AST- 82.9 g/m², C-79.6 g/ m², p-0.001), but better diastolic function and filling time (FT) (HST-56.2 %, AST-55.9 %,C-51.7%, p-0.02) than controls; systolic function was similar. However, HST group had increased parameters of intraventricular synchrony as compared to AST and C groups, due to lower time-to-peak peak systolic longitudinal velocity or strain in the inferior and lateral walls ( SD-Ts (ms): HST-22.5, AST-19.1, C-16.4, p-0.01; SD-Sl (ms): HST- 37.1, AST-32.6, C-30.6, p-0.02). A cut-off value of 19.5 ms for SD-Ts and of 32.5 ms for SD-Sl were predictive for “malignant” ERS (AUC 0.87 and 0.93, respectively). Conclusions: Subjects with HST have increased parameters of intraventricular dissynchrony compared to AST and C groups. These subtle changes suggest particular properties of repolarization in infero-lateral segments and might be used as a screening tool to define ERS subjects at risk of sudden cardiac death.