Aim: Urinary tract infection (UTI) is one of the most common diseases of urogenital tract in children. Detecting predisposing factors for UTI takes an important place in managing patients with UTI. Recently, a few studies emphasized on idiopathic hypercalciuria (IH) as a predisposing factor for UTI and dysfunctional voiding. Therefore, we carried out a survey to find out whether non-calculus IH is a contributing factor in children with the first attack of pyelonephritis. Materials and Methods: This is a case-control study carried out on 60 children aged 2-11 years admitted at St Al-Zahra hospital, Isfahan, Iran, with the first episode of upper UTI and 200 age-and gender-matched normal healthy children between September 2003 and February 2005. We used second fasting spot urine sample to measure calcium and creatinine. Two urine samples were obtained one week apart to increase the accuracy of measurement. All samples were collected after at least 6 weeks of completing the treatment course of pyelonephritis. Ultrasound examination and VCUG were performed in all patients before entering the survey as case group to rule out obstruction and VUR. Results: Mean age of case and control group were 4.86 ± 3.08 years and 4.22 ± 2.9 years, respectively. The mean calcium to creatinine ratio (Ca/Cr) in case and control group were 0.308 ± 0.21 and 0.208 ± 0.12 mg/ mg, respectively, P < 0.001. The difference between the mean values of these two groups was significant only in age group ≤6 years, P < 0.0001 and odds ratio was 2.1 (95% CI 1.03-7.8). After determining the mean values of urine Ca/Cr ration according to both age groups and gender, it was cleared that only significant difference was related to male <6 years. Conclusion: The likelihood of hypercalciuria should be assessed especially in male children with UTI and without any urinary tract obstruction.