Hypercalciuria is a common problem causing symptoms such as abdominal pain, hematuria and enuresis, and leading to stone formation. It results from a renal tubular calcium "leak" or intestinal hyper-reabsorption of calcium. This study was performed to determine whether renal functional impairment was present in children with hypercalciuria. The study group comprised 298 children who were screened for hypercalciuria by means of urinary calcium/creatinine (UCa/UCr) ratio. The renal functions of 18 children (6.4%) detected as having hypercalciuria with Ca/Cr ratios of greater than 0.18 in their spot urines were evaluated. Results were compared with those of the healthy control group. The rate of hypercalciuria did not very significantly between the boys and girls (p > 0.05). The mean value of daily calcium excretion was 6.42 + 3.93 mg/kg/day in the children with hypercalciuria, which was significantly different from that of the control group (p < 0.01). When the values of creatinine, osmolar and free water clearances, fractional excretion of sodium and tubular reabsorption of phosphorus were compared between the patient and control groups, the difference was not significant (p > 0.05). Urinary N-acetyl-beta-D-glucosaminidase (NAG) excretion, which was described as the creatinine ratio, was significantly higher in the children with hypercalciuria. These findings suggest that in the presence of normal renal functional studies in children with hypercalciuria, tubular injury can be detected by NAG, which is a more sensitive marker of renal tubular injury.