• Pulmonary and renal responses to furosemide were evaluated in ten infants with stage III-IV bronchopulmonary dysplasia. Furosemide was given intravenously for two doses, 1 and 2 mg/kg, at approximately 24-hour intervals. The following indices were evaluated in series before and after entry Into study: clinical respiratory distress syndrome score; blood pH; partial pressure of arterial carbon dioxide; alveolar-arterial oxygen gradient; urine output; glomerular filtration rate; fractional excretion of filtered sodium, chloride, potassium, and calcium; osmolar clearance; water clearance; and concentrations of serum electrolytes and calcium. A significant decrease in the respiratory distress syndrome score and in the partial pressure of arterial carbon dioxide was seen only transiently at two hours following each dose. There was no significant improvement in the alveolar-arterial oxygen gradient during the study. Furosemide induced diuresis and urinary excretion of sodium, chloride, potassium, and calcium. A significant decrease in serum chloride and potassium concentrations was seen at 48 hours after entry intostudy; serum sodium and calcium concentrations remained unchanged. This study has demonstrated that furosemide administration has only a short-term effect in the lung but has a potential for long-term complication on electrolytes and calcium balance in infants with well-established stage III-IV bronchopulmonary dysplasia.