Severe neonatal Ebstein's anomaly (EA) and tricuspid valve dysplasia (TVD) are associated with high perinatal morbidity and mortality. The authors recently demonstrated left ventricular (LV) dysfunction and dyssynchrony to be prevalent in affected newborns and to contribute to poor outcomes. The aim of this study was to investigate the impact of patent ductus arteriosus (PDA) closure, spontaneous or surgical ligation, or right ventricular exclusion (Starnes procedure) on LV performance in neonatal EA and TVD. Neonates with EA or TVD encountered from 2004 to 2018 at three institutions were identified. Pre-and postoperative LV function was assessed using two-dimensional, Doppler-derived deformation (six-segment vector velocity imaging) and two measures of mechanical dyssynchrony (the SD of time to peak and global dyssynchrony index), and values were compared using paired t test analysis or the Wilcoxon rank sum test. Before the intervention, LV function was impaired in the PDA (n=18) and Starnes (n=6) groups and was similar between groups. After PDA closure, LV performance did not change. After the Starnes procedure, however, LV function, including synchrony, improved significantly: fractional area change from 45±5% to 58±8% (P=.003), global circumferential strain from -18.2±5.0% to -32.5±5.5% (P=.01), cardiac index from 1.9±0.3 to 3.9±1.5L/min/m2 (P=.05), and circumferential strain dyssynchrony (dyssynchrony index from 0.19±0.09 to 0.04±0.02 [P=.009] and SD of time to peak from 59.8±18.5 to 29.9±8.2 [P=.02]). The Starnes procedure results in early improvements in LV dysfunction and dyssynchrony, not observed after PDA closure in neonatal severe EA and TVD, which may benefit critically unwell neonates.