Long term furosemide therapy has been recommended for neonatal chronic lung disease. 28 preterm infants (986±60 gm) with stable or worsening chronic lung disease due to bronchopulmonary dysplasia or chronic pulmonary insufficiency of the premature were treated with 1-2 mg/kg q 6 hours of furosemide. Infants entered this prospective study if they required mechanical ventilation for >2 wks or for FiO2 of>.4,>.3 or >.21 at 3, 4 or 6 wks of age, respectively. Infants were evaluated using a computerized score based on peak ventilator pressures, end expiratory pressures, ventilator rate, FiO2 and PaCO2. Each infant acted as his own control. If he responded initially, therapy was discontinued after 7 days and he was evaluated for deterioration. 20/28 infants showed significant improvement (p<.010) when treated with furosemide. There were no significant differences between responders (n=20) and nonresponders (n=8) in mean gestational age (28.2 vs 28.5 wks), birth weight, male-female distribution, age at start of study, or type of chronic lung disease. 12/20 responders had a significant deterioration in their respiratory scores when therapy was discontinued. 21/28 received a second trial of furosemide: 14/21 responded to retreatment. An infant's response in the second trial was independent of his response in the first trial. Brief periods of furosemide therapy may be useful in the management of chronic lung disease in some preterm infants, but prolonged continuous use needs further evaluation.