© 2021, Asociacion Regional de Dialisi y Transplantes Renales de Capital Federal y Provincia de Buenos Aires. All rights reserved.Objective: The objective of this study was to assess serum and urinary magnesium levels in children who have chronic kidney disease stages 1-3. Methods: Eighty-seven patients who were followed at pediatric nephrology department for chronic kidney disease were included in the study. Age, gender, magnesium, dietary magnesium, and creatinine levels, and fractionated magnesium excretion for all cases were recorded. Patients with chronic kidney disease and control groups were compared in terms of these data. Results: Thirty-nine cases with chronic kidney disease were stage 1, 26 were stage 2, and 22 were stage 3. Average age was 9.9 ± 2.8 years in the control group and 10.2 ± 2.6 years in the chronic kidney disease group. The serum magnesium levels were significantly higher in the stage 3 group than in the control group (P<0.001). Also, in stage 3, fractionated magnesium excretion levels were higher compared to the control group (P<0.001). Conclusion: In chronic kidney disease with advancing renal failure, hypermagnesemia is frequently seen. Serum magnesium levels should be measured periodically in all the children with chronic kidney disease stage 3 to investigate magnesium abnormalities and assess clinical results.