Background: An increased spot urine Na/K ratio (UNa/K) has been found to be related to urinary stone disease in adults with a history of nephrolithiasis and in children with idiopathic hypercalciuria (HC). However, the respective role played by Na and K excretion in the rise of the UNa/K in growing individuals is not well clarified. Methods: The urinary excretion of Na and K was evaluated in fasting morning and 24-hour urine samples of 37 consecutive children with HC and of 21 previously HC children who were normocalciuric at the time of the study (ExHC). None of them had received any dietary or specific drug prescription. Results: In the HC and in the ExHC group, respectively, the Na excretion was 4 ± (SD) 2.4 and 2.9 ± 1.3 mmol/kg/day (p = 0.009); the K excretion was 1.1 ± 0.4 and 1.2 ± 0.7 mmol/kg/day (p = 0.86); the fasting UNa/K was 3 ± 1.6 and 2.1 ± 1 mmol/mmol (p = 0.044), and the 24-hour UNa/K was 4.2 ± 3.9 and 2.8 ± 1.5 mmol/ mmol (p = 0.045). The 24-hour UNa/K was significantly higher than the fasting UNa/K in both HC (p = 0.002) and ExHC (p = 0.002) subjects. The 24-hour UNa/K significantly increased with age in both HC (p = 0.02) and ExHC (p = 0.015) children. The K excretion significantly decreased with age in HC (p = 0.0001) and ExHC (p = 0.005) subjects, as well as with body weight gain in HC (p = 0.005) and ExHC (p = 0.0001) children and with increasing body height in HC (p = 0.006) and ExHC (p = 0.001) subjects. In neither group was the K excretion significantly related to body mass index Z score nor to height Z score. No significant relation resulted between Na excretion and age, body weight and height, and body mass index Z score and height Z score. Conclusions: HC children have a higher Na excretion as well as a higher fasting and 24-hour UNa/K than ExHC children, but no different K excretion. Meals are accompanied by a significant rise in UNa/K. The rise in UNa/K with age is mostly due to a decrease in K excretion which possibly depends on childhood body growth.