Objective: To compare spirometry and volumetric capnography (VCap) to determine if the capnographic values add more information about early lung disease in cystic fibrosis (CF) patients.Methods: This was a cross-sectional study involving CF patients: Group I (42 patients, 6-12 years of age); and Group II (22 patients, 13-20 years of age). The corresponding control groups were comprised of 30 and 50 healthy subjects, respectively. Forced vital capacity (FVC), forced expiratory volume in one second (FEV 1 ), and the FEV 1 /FVC ratio was determined by spirometry. Using VCap, we measured peripheral oxygen saturation (SpO 2 ), respiratory rate (RR), inspiratory time (IT), expiratory time (ET), and the phase III slope normalized by expiratory volume (phase III slope/Ve). Results:In comparison with control groups, all CF patients presented higher phase III slope/Ve values (p < 0.001) independent of the pulmonary disease stage. The phase III slope/Ve was significantly higher in the 24 patients who presented normal spirometry results (p = 0.018). The Group II patients showed lower FVC, FEV 1 , FEV 1 /FVC (p < 0.05), and also lower SpO 2 values (p < 0.001) when compared with Group I patients. In comparison with Control Group II, the Group II patients presented higher RR (p < 0.001), and lower IT and ET values (p < 0.001). Conclusions:Compared to the controls, all studied CF patients showed an increase in phase III slope/Ve values. VCap identified the heterogeneity of the ventilation distribution in the peripheral airways of CF patients who presented normal spirometry.J Pediatr (Rio J). 2012;88(6)