In adult patients with chronic obstructive pulmonary disease, there is a gradient between end-tidal carbon dioxide (EtCO(2)) and arterial carbon dioxide pressure (PaCO(2)), and the slope of the ascending phase of the capnogram is decreased due to obstruction. Corresponding data are lacking in infants with bronchopulmonary dysplasia (BPD). To compare PCO(2) -EtCO(2) gradient and capnogram shape in two groups of spontaneously breathing preterm subjects: infants with BPD and infants without respiratory disease (controls). Capnography was performed at 36 weeks postmenstrual age in 20 infants (12 BPD with oxygen dependency, 8 controls). Respiratory rate and the components of the respiratory cycle were measured. The PCO(2) -EtCO(2) gradient was calculated using EtCO(2) values and simultaneously sampled capillary values (PcCO(2)). Capnograms were compared between groups. In BPD subjects, respiratory rate was increased (60 ± 16 bpm vs 43 ± 16 bpm, P = 0.009); a widened PcCO(2) -EtCO(2) gradient was observed (13 ± 4 mmHg vs 0 ± 7 mmHg, P = 0.0013); the ascending phase of the capnogram was not decreased, whereas the initial inspiratory phase was prolonged (0.32 ± 0.05 vs 0.24 ± 0.04, P = 0.001). Compared with healthy infants, a higher PcCO(2) -EtCO(2) gradient was observed in infants with BPD, suggesting that ventilation-perfusion mismatch may be present in these infants. The capnogram did not exhibit the characteristic shape of airway obstruction. Copyright © 2011 Wiley-Liss, Inc.