Angiotensin-converting enzyme inhibitors (ACEi) for renin-angiotensin-aldosterone system (RAAS) blockade are routinely used to slow CKD progression. However, vitamin D may also promote renoprotection by suppressing renin transcription through cross-talk between RAAS and vitamin D-fibroblast growth factor-23 (FGF-23)-Klotho pathways. To determine whether vitamin D levels influence proteinuria and CKD progression in children, we performed a post hoc analysis of the Effect of Strict Blood Pressure Control and ACE Inhibition on Progression of CKD in Pediatric Patients (ESCAPE) cohort. In 167 children (median eGFR 51 ml/min per 1.73 m(2)), serum 25-hydroxyvitamin D (25(OH)D), FGF-23, and Klotho levels were measured at baseline and after a median 8 months on ACEi. Children with lower 25(OH)D levels had higher urinary protein/creatinine ratios at baseline (P=0.03) and at follow-up (P=0.006). Levels of 25(OH)D and serum vitamin D-binding protein were not associated, but 25(OH)D <= 50 nmol/L associated with higher diastolic BP (P=0.004). ACEi therapy also associated with increased Klotho levels (P<0.001). The annualized loss of eGFR was inversely associated with baseline 25(OH)D level (P<0.001, r=0.32). Five-year renal survival was 75% in patients with baseline 25(OH)D >= 50 nmol/L and 50% in those with lower 25(OH) D levels (P<0.001). This renoprotective effect remained significant but attenuated with ACEi therapy (P=0.05). Renal survival increased 8.2% per 10 nmol/L increase in 25(OH)D (P=0.03), independent of eGFR; proteinuria, BP, and FGF-23 levels; and underlying renal diagnosis. In children with CKD, 25(OH)D >= 50 nmol/L was associated with greater preservation of renal function. This effect was present but attenuated with concomitant ACEi therapy.