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Purpose: Nephrocalcinosis (NC) is frequently observed in premature infants. Smallscale studies have suggested that NC adversely affects renal function; however, the etiologic factors are still unclear. This prospective observational study aimed to identify the factors that influence the development of NC, through urine analysis. Methods: In total, 99 preterm infants (gestational age <34 weeks) diagnosed with NC in the neonatal intensive care unit (NICU) from October 2010 to March 2014 were evaluated. Data regarding perinatal characteristics, respiratory support, total parenteral nutrition (TPN), and use of nephrotoxic drugs were analyzed. After an ultrasonographic diagnosis of NC, the infants were subjected to biweekly urine tests along with ultrasonographic follow-ups until the resolution of NC, in the outpatient department. Results: NC was diagnosed in 23% (99/432) of the preterm infants admitted to the NICU. Their median gestational age and birth weight were 28+3 (range:23+5-35+2) weeks and 1,120 (range: 560-1,950) g, respectively. NC was diagnosed an average of 26.4±2.8 (range: 2-82) days after birth, and the corrected gestational age at that time was 32.4± 2.0 weeks. Preterm infants with NC had hyperoxaluria (oxalate/Cr=4.1 [oxalate/Cr< 0.3]), and low urinary citrate levels (citrate/Cr=0.03 [citrate/Cr>0.51]). The follow-up rate was 52% (27/52) and symptoms in none of the infants had progressed to nephrolithiasis. In the infants that were followed up, NC was resolved at a mean age of 7.7 (range: 2-32) months. Conclusion: Our results suggest that hyperoxaluria is a significant risk factor for the development of NC. [ABSTRACT FROM AUTHOR]