Right ventricular (RV) pressure overload has a vastly different clinical course in children with idiopathic pulmonary arterial hypertension (iPAH) than in children with pulmonary stenosis (PS). While RV function is well recognized as a key prognostic factor in iPAH, adverse ventricular-ventricular interactions and LV dysfunction are less well characterized and the pathophysiology is incompletely understood. We compared ventricular-ventricular interactions as hypothesized drivers of biventricular dysfunction in pediatric iPAH versus PS Eighteen iPAH, 16 PS patients and 18 age- and size-matched controls were retrospectively studied. Cardiac cycle events were measured by M-mode and Doppler echocardiography. Measurements were compared between groups using ANOVA with post hoc Dunnet's or ANCOVA including RV systolic pressure (RVSP; iPAH 96.8 ± 25.4 mmHg vs. PS 75.4 ± 18.9 mmHg; P = 0.011) as a covariate. RV-free wall thickening was prolonged in iPAH versus PS, extending beyond pulmonary valve closure (638 ± 76 msec vs. 562 ± 76 msec vs. 473 ± 59 msec controls). LV and RV isovolumetric relaxation were prolonged in iPAH (P < 0.001; LV 102.8 ± 24.1 msec vs. 63.1 ± 13.7 msec; RV 95 [61-165] vs. 28 [0-43]), associated with adverse septal kinetics; characterized by rightward displacement in early systole and leftward displacement in late RV systole (i.e., early LV diastole). Early LV diastolic filling was decreased in iPAH (73 ± 15.9 vs. PS 87.4 ± 14.4 vs. controls 95.8 ± 12.5 cm/sec; P = 0.004). Prolonged RVFW thickening, prolonged RVFW isovolumetric times, and profound septal dyskinesia are associated with interventricular mechanical discoordination and decreased early LV filling in pediatric iPAH much more than PS These adverse mechanics affect systolic and diastolic biventricular efficiency in iPAH and may form the basis for worse clinical outcomes. We used clinically derived data to study the pathophysiology of ventricular-ventricular interactions in right ventricular pressure overload, demonstrating distinct differences between pediatric pulmonary arterial hypertension (iPAH) and pulmonary stenosis (PS). Altered timing of right ventricular free wall contraction and profound septal dyskinesia are associated with interventricular mechanical discoordination and decreased early LV filling in iPAH much more than PS These adverse mechanics affect systolic and diastolic biventricular efficiency, independent of right ventricular systolic pressure.