A reduction of lung recoil pressure could aggravate any airflow limitation that might be present in patients with cystic fibrosis (CF) or asthma. In a group of 22 children and young adults (8 with cystic fibrosis, 8 with asymptomatic asthma, and 6 healthy controls) aged 8-24 years, lung recoil pressure (Pst) at 100%, 90%, and 60% of TLC and static lung compliance (Cst) were measured using an esophageal balloon. The indices of airflow limitation, including maximal expiratory flow at 25% VC (Vmax25), forced expiratory volume in 1 second (FEV1), and specific airway conductance (sGaw), were also measured. In all patients, Vmax25 was reduced, the airway obstruction being more pronounced in patients with CF. Pst was reduced in CF and asthma, again more in the patients with CF. Cst was normal in both groups because the pressure volume curve was shifted up and to the left. There were significant correlations between Pst at 60%, 90%, and 100% of TLC and both Vmax25 and FEV1 (P < 0.01). sGaw correlated with Pst90 and Pst60 (r = 0.47 and 0.53, respectively; P < 0.05 for both). No correlation was found between Cst and Pst at any lung volume. No correlations were observed between Cst and Vmax25, FEV1, or sGaw. These results suggest that loss of elastic lung recoil pressure is a factor in airflow limitation of children and young adults with CF or asthma.