The aim of this study was to determine (1) whether ibuprofen treatment in very preterm infants causes an increase in the renal water channel aquaporin-2 (AQP2) activity in the collecting duct via prostaglandin synthesis inhibition and (2) whether AQP2 activity remains disturbed long after ibuprofen treatment has ended. This was a prospective study involving premature infants with a gestation age of 27-31 weeks who received treatment between December 2005 and August 2006 in a tertiary Neonatal Intensive Care Unit. Each ibuprofen-treated infant was matched to two controls. Renal glomerular and tubular function were evaluated weekly for 1 month, and urinary AQP2 was measured by immuno-dotting. In total, 166 longitudinal samples were analyzed in 36 infants. Median [interquartile range] gestational age and birthweight were 28 [27.0-29.5] weeks and 1160 [1041-1242] g, respectively. Perinatal factors were similar in both groups. Urine output was significantly decreased in the ibuprofen-treated infants during the treatment. The urinary AQP2 level decreased significantly from day 2 to day 7 in both groups and was similar thereafter for the first month of life in ibuprofen-treated and control groups. Based on our results, we conclude that ibuprofen-induced oligo-anuria is not associated with a change in AQP2 activity and that ibuprofen does not affect AQP2 activity during the first month of life in very preterm neonates.