<p><strong>Rationale &amp; Objective: </strong>Chronic kidney disease (CKD) in children is associated with cognitive dysfunction that affects school performance and quality of life. The relationship between CKD-mineral and bone disorder and cognitive function in children is unknown.</p>

<p><strong>Study Design: </strong>Observational study.</p>

<p><strong>Participants: </strong>702 children enrolled in the Chronic Kidney Disease in Children (CKiD) Study.</p>

<p><strong>Predictors: </strong>Plasma fibroblast growth factor 23 (FGF-23), parathyroid hormone (PTH), calcium, phosphorus, 25 hydroxyvitamin D (25[OH]D), and 1,25 dihydroxyvitamin D (1,25[OH]D).</p>

<p><strong>Outcomes: </strong>Neurocognitive tests of intelligence, academic achievement, and executive functions.</p>

<p><strong>Analytical Approach: </strong>Linear regression models to analyze the cross-sectional associations between logFGF-23, 25(OH)D, 1,25(OH)D, PTH, calcium, and phosphorus scores and the cognitive test scores of interest after adjustment for demographics, blood pressure, proteinuria, and kidney function.</p>

<p><strong>Results: </strong>At baseline, median age was 12 (95% CI, 8.3, 15.2) years and estimated glomerular filtration rate was 54 (40.5, 67.8) mL/min/1.73&nbsp;m. In fully adjusted analyses, 25(OH)D, 1,25(OH)D, PTH, calcium, and phosphorus scores did not associate with cognitive test scores. In fully adjusted analyses, logFGF-23 was associated with abnormal test scores for attention regulation (&nbsp;&lt;&nbsp;0.05); specifically, Conners' Continuous Performance Test II Errors of Omission (β&nbsp;=&nbsp;2.3&nbsp;[1.0, 3.6]), Variability (β=1.4 [0.4,&nbsp;-2.4]), and Hit Reaction Time (β&nbsp;=&nbsp;1.3&nbsp;[0.2, 2.4]). Children in the highest FGF-23 tertile group had 7% and 9% greater cognitive risk for&nbsp;Hit Reaction Time and Errors of Omission compared with those in the lowest tertile, respectively. In fully adjusted analyses, higher FGF-23 tertile was associated with increased cognitive risk (&nbsp;&lt;&nbsp;0.05) for Errors of Omission (β&nbsp;=&nbsp;0.4&nbsp;[0.1,&nbsp;0.7]) and Hit Reaction Time (β&nbsp;=&nbsp;0.4&nbsp;[0.1, 0.7]).</p>

<p><strong>Limitations: </strong>The study does not assess the cumulative&nbsp;effects of FGF-23 excess on cognitive function over time. Within-population stratified analyses were not performed due to limited sample size.</p>

<p><strong>Conclusions: </strong>In children with CKD, higher plasma FGF-23 level is associated with lower performance in targeted tests of executive function, specifically attention regulation, independent of glomerular filtration rate.</p>