Renal function wa assessed in 20 newborn infants with idiopathic respiratory distress syndrome and compared to that of 10 neonates without respiratory distress or renal disease. Inulin and PAH clearances were markedly depressed in neonates with RDS as compared to controls (5.9 +/- 0.6 vs 9.3 +/- 0.8 ml/min/m2 (p less than 0.01) and 13.5 +/- 2.0 vs 23.2 +/- 1.2 ml/min/m2 ( p less than 0.01), respectively). The impairment of inulin and PAH clearances correlated with the severity of the pulmonary disease. Improvement of the respiratory distress was followed by a progressive rise of inulin and PAH clearances toward normal values. Intravenous administration of hypertonic mannitol in three patients resulted in an immediate increase in urine flow and inulin and PAH clearances. It is concluded that a state of acute, reversible, renal insufficiency can occur in the acute phase of idiopathic respiratory distress syndrome.