INTRODUCTION: Adults with childhood-onset chronic kidney disease (CKD) have increased risk of cardiovascular disease. First-phase ejection fraction (EF1) a novel measure of early systolic function may be a more sensitive marker of left ventricular dysfunction than other markers in children with CKD. OBJECTIVE: To examine whether EF1 is reduced in children with CKD. METHODS: Children from 4C and HOT-KID studies were stratified according to estimated glomerular filtration rate (eGFR). EF1 was calculated from the fraction of LV volume ejected up to the time of peak aortic flow velocity. RESULTS: EF1 was measured in children aged 10.9±3.7 (mean±SD) years, 312 with CKD and 63 healthy controls. EF1 was lower, whilst overall EF was similar in those with CKD compared to controls and decreased across stages of CKD (29.3±3.7%, 23.5±4.5%, 19.8±4.0%, 18.5±5.1% and 16.7±6.6% in controls, CKD 1, 2, 3 and ≥4, respectively, p<0.001). The relationship of EF1 to eGFR persisted after adjustment for relevant confounders (p<0.001). The effect size for association of measures of LV structure or function with eGFR (SD change per unit change in eGFR) was greater for EF1 (β=0.365, p<0.001) than other measures: LVMi (β=-0.311), RWT (β=-0.223), E/e' (β=-0.147), and e' (β=0.141) after adjustment for confounders in children with CKD. CONCLUSIONS: Children with CKD exhibit a marked and progressive decline in EF1 with falling eGFR. This suggests that EF1 is a more sensitive marker of LV dysfunction when compared to other structural or functional measures and early LV systolic function a key feature in the pathophysiology of cardiac dysfunction in CKD.Read less <