• We investigated the antidiuretic hormone (ADH) response in 12 infants with bronchopulmonary dysplasia during acute respiratory distress. All of the infants had hypoxemia with air-trapping in the chest at the time of admission to the hospital. None had documented infection. There was a dramatic increase in the plasma levels of ADH during acute respiratory distress, with a subsequent reduction of levels toward normal when the respiratory distress decreased to the preadmission well state. Three of 12 infants manifested hyponatremia at 24 hours after admission, with two of them exhibiting persistent hypertension for up to three days. The mechanism for elevated ADH levels is air-trapping in the chest, causing pulmonary hypovolemia and decreased left atrial filling and/or decreased transmural pressure of the left atrium.