Offspring of diabetic rats have reduced urinary calcium and magnesium excretion compared with offspring of controls; these differences persist up to 16 weeks after birth, a time equivalent to young adulthood in humans. Objectives: To test the hypothesis that urinary calcium and magnesium excretion would be lower in children born to mothers with insulin dependent diabetes mellitus (ChMIDDM) than those born to nondiabetic mothers. Methods: Concentrations of calcium, magnesium, sodium, and creatinine were measured in first void spot urine samples collected from 45 (28 male; median age 9.6 years) ChMIDDM and 127 (58 male; median age 11.3 years) controls. Analysis of covariance was used to test for differences in urinary calcium to creatinine ratios (UCa/Cr), magnesium to creatinine ratios (UMg/Cr), and log sodium to creatinine ratios (logUNa/Cr) between controls and ChMIDDM after allowing for the effects of sex and age. Results: UCa/Cr (difference - 0.10, 95% confidence interval (CI) - 0.19 to - 0.01; p = 0.03) and UMg/Cr (difference - 0.15, 95% CI - 0.22 to - 0.08; p < 0.0001) were lower in ChMIDDM than controls. However, logUNa/Cr did not differ between Ch MIDDM and controls (difference - 0.14, 95% CI - 0.33 to 0.05; p = 0.1). The daily estimated intake of magnesium, sodium, and protein were significantly higher and that of calcium non-significantly higher in ChMIDDM than controls. In ChMIDDM, UCa/Cr and UMg/Cr were not related to diabetic control of mothers. Conclusions: Results of this study provide the first evidence that in humans, as in rats, there is modification of renal Ca and Mg handling in ChMIDDM, which persists well into childhood. Offspring of diabetic rats have reduced urinary calcium and magnesium excretion compared with offspring of controls; these differences persist up to 16 weeks after birth, a time equivalent to young adulthood in humans. Objectives: To test the hypothesis that urinary calcium and magnesium excretion would be lower in children born to mothers with insulin dependent diabetes mellitus (ChMIDDM) than those born to nondiabetic mothers. Methods: Concentrations of calcium, magnesium, sodium, and creatinine were measured in first void spot urine samples collected from 45 (28 male; median age 9.6 years) ChMIDDM and 127 (58 male; median age 11.3 years) controls. Analysis of covariance was used to test for differences in urinary calcium to creatinine ratios (UCa / Cr), magnesium to creatinine ratios (UMg / Cr) sodium to creatinine ratios (logUNa / Cr) between controls and ChMIDDM after allowing for the effects of sex and age. Results: UCa / Cr (difference - 0.10, 95% confidence interval 9 to-0.01; p = 0.03) and UMg / Cr (difference - 0.15, 95% CI -0.22 to -0.08; p <0.0001) were lower in ChMIDDM than controls. However, logUNa / Cr did not differ between Ch MIDDM and The daily estimated intake of magnesium, sodium, and protein were significantly higher and that of calcium non-significantly higher in ChMIDDM than controls. In ChMIDDM, UCa (difference - 0.14, 95% CI -0.33 to 0.05; / Cr and UMg / Cr were not related to diabetic control of mothers. Conclusions: Results of this study provide the first evidence that in humans, as in rats, there is modification of renal Ca and Mg handling in ChMIDDM, which persists well into childhood .