Babies with chronic bronchopulmonary dysplasia (BPD) can sometimes develop pallor, systemic and pulmonary edema, oliguria, and hyponatremia not attributable to cardiopulmonary or renal impairment. These signs and symptoms might, however, be explained by inappropriate control of vasopressin secretion. To test this hypothesis, we measured plasma vasopressin and osmolality, serum sodium and potassium concentrations, urine output and osmolality, and free water clearance in 26 normoxic infants with BPD aged 1–4 months. All of these infants required supplemental oxygen (FiO20.41 ± 0.03, mean ± 1 SE) to maintain O2saturation of > 88%, and six infants also required mechanical ventilation. As controls, 10 infants of similar age but without BPD were also studied. None of the infants had been discharged from the nursery and was receiving any medications, and all were clinically stable when studied. Compared to control infants, infants with BPD had significantly elevated plasma vasopressin concentrations (control 5.2 ± 0.9 pg/ml; BPD 42.4 ± 5.1; mean ± SE,p> 0.05). Moreover, infants with BPD had hyponatremia and hypotonic plasma, and both urine output and free water clearance were significantly reduced. These data suggest that some infants with chronic BPD have elevated vasopressin levels that are functionally significant. We speculate that excessive stimulation of vasopressin secretion may explain some of the pulmonary and nonpulmonary signs and symptoms in infants with chronic BPD.