We studied the effects of furosemide on pulmonary mechanics in 10 infants with bronchopulmonary dysplasia aged 41 +/- 1 (SE) weeks post-conception, gestational age at birth 30 +/- 1 wk, birth weight 1370 +/- 200 gm. Thoracic gas volume, airways resistance, and specific airway conductance were measured in an infant body pressure plethysmograph during quiet breathing. Dynamic pulmonary compliance was measured using an esophageal balloon. Infants with BPD had greater Raw, lower SGaw, and lower Cdyn than did 16 normal control infants. Within one hour after administration of furosemide 1 mg/kg IV to infants with BPD, Raw fell 36 +/- 13%, SGaw increased 84 +/- 22%, and Cdyn increased 54 +/- 13%; TGV did not change. Diuretic treatment of BPD in infants is associated with rapid, short-term improvement in Raw and Cdyn.