Background: Chronic right ventricular (RV) pacing is associated with deleterious effects on cardiac function. Objective In an observational multicentre study in children with isolated atrioventricular (AV) block receiving chronic ventricular pacing, the importance of the ventricular pacing site on left ventricular (LV) function was investigated. Methods: Demographics, maternal autoantibody status and echocardiographic measurements on LV end-diastolic and end-systolic dimensions and volumes at age <18 years were retrospectively collected from patients undergoing chronic ventricular pacing (>1 year) for isolated AV block. LV fractional shortening (LVFS) and, if possible LV ejection fraction (LVEF) were calculated. Linear regression analyses were adjusted for patient characteristics. Results: From 27 centres, 297 children were included, in whom pacing was applied at the RV epicardium (RVepi, n=147), RV endocardium (RVendo, n=113) or LV epicardium (LVepi, n=37). LVFS was significantly affected by pacing site (p=0.001), and not by maternal autoantibody status (p=0.266). LVFS in LVepi (39±5%) was significantly higher than in RVendo (33±7%, p<0.001) and RVepi (35±8%, p=0.001; no significant difference between RV-paced groups, p=0.275). Subnormal LVFS (LVFS<28%) was seen in 16/113 (14%) RVendo-paced and 21/147 (14%) RVepi-paced children, while LVFS was normal (LVFS≥28%) in all LVepi-paced children (p=0.049). These results are supported by the findings for LVEF (n=122): LVEF was <50% in 17/69 (25%) RVendo- and in 10/35 (29%) RVepi-paced patients, while LVEF was ≥50% in 17/18 (94%) LVepi-paced patients. Conclusion: In children with isolated AV block, permanent ventricular pacing site is an important determinant of LV function, with LVFS being significantly higher with LV pacing than with RV pacing.