Nephronic Reduction After Neonatal Acute Renal Failure in Preterm

The aim of this study is to evaluate the signs of nephronic reduction in preterm infants who have presented neonatal acute renal failure. The investigators hypothesize that signs of nephronic reduction would appear earlier in former preterm with neonatal acute renal failure than in control preterm infants.

Inclusion Criteria: * For both group of cases (50 infants having presented acute renal failure in preterm) AND group of control cases (25 infants without this) :former preterm infant born before 33 weeks of gestational age between january 2003 and june 2010 and hospitalized in the neonatal intensive care unit of Nantes University Hospital. * Specific to cases: neonatal acute renal failure: serum creatinine\>130 micromol/l from the third day of life. * Control cases: no such renal dysfunction Exclusion Criteria: * no parental consent * other causes of renal failure: congenital uropathy, congenital nephropathy * congenital cardiopathy, polymalformative syndrome