This study was designed to use volumetric capnography to evaluate the breathing pattern and ventilation inhomogeneities in patients with chronic sputum production and bronchiectasis and to correlate the phase 3 slope of the capnographic curve to spirometric measurements. Twenty-four patients with cystic fibrosis (CF) and 21 patients with noncystic fibrosis idiopathic bronchiectasis (BC) were serially enrolled. The diagnosis of cystic fibrosis was based on the finding of at least two abnormal sweat chloride concentrations (iontophoresis sweat test). The diagnosis of bronchiectasis was made when the patient had a complaint of chronic sputum production and compatible findings at high-resolution computed tomography (HRCT) scan of the thorax. Spirometric tests and volumetric capnography were performed. The 114 subjects of the control group for capnographic variables were nonsmoker volunteers, who had no respiratory symptoms whatsoever and no past or present history of lung disease. Compared with controls, patients in CF group had lower SpO(2) (P < 0.0001), higher respiratory rates (RR) (P < 0.0001), smaller expiratory volumes normalized for weight (V(E)/kg) (P < 0.028), smaller expiratory times (Te) (P < 0.0001), and greater phase 3 Slopes normalized for tidal volume (P3Slp/V(E)) (P < 0.0001). Compared with controls, patients in the BC group had lower SpO(2) (P < 0.0001), higher RR (P < 0.004), smaller V(E)/kg (P < 0.04), smaller Te (P < 0.007), greater P3Slp/V(E) (P < 0.0001), and smaller VCO(2) (P < 0.0002). The pooled data from the two patient groups compared with controls showed that the patients had lower SpO(2) (P < 0.0001), higher RR (P < 0.0001), smaller V(E)/kg (P < 0.05), smaller Te (P < 0.0001), greater P3Slp/V(E) (P < 0.0001), and smaller VCO(2) (P < 0.0003). All of the capnographic and spirometric variables evaluated showed no significant differences between CF and BC patients. Spirometric data in this study reveals that the patients had obstructive defects with concomitant low vital capacities and both groups had very similar abnormalities. The capnographic variables in the patient group suggest a restrictive respiratory pattern (greater respiratory rates, smaller expiratory times and expiratory volumes, normal peak expiratory flows). Both groups of patients showed increased phase III slopes compared with controls, which probably indicates the presence of diffuse disease of small airways in both conditions leading to inhomogeneities of ventilation.