A computerized pneumotachometric multiple breath N2-washout method for assessment of the volume of trapped gas in the lungs (VTGN2) is presented. The VTGN2 is measured as the volume of air mobilized from nonventilated lung spaces by five maximal breaths after a washout performed until the end-tidal nitrogen fraction is 0.02. The method demonstrated a good instrumental precision and the reproducibility of VTGN2 recordings in normal subjects was equal to that achieved with a previous VTGN2 method based on gas collection in bags. It was confirmed that gas trapping occurs in normal children during tidal breathing at functional residual capacity. In normal subjects VTGN2 was directly related to lung size. In 69 healthy children and adolescents VTGN2 showed a good correlation with vital capacity (r = 0.85; P < 0.001), of which it comprised 1.7 +/- 0.4% (mean +/- SD). Patients with bronchial asthma or cystic fibrosis investigated had pathological gas trapping with only a few exceptions; in several cases despite normal results at forced expiratory spirometry. The relative response of VTGN2 (reflecting peripheral airway obstruction) and forced expiratory volume in one second (FEV1) (reflecting conditions in central airways) to beta 2-agonist inhalation among the patients with asthma was variable, indicating that bronchial obstruction is not uniformly distributed along the bronchial tree. Measurements of VTGN2 can be easily performed in children from 7 years of age with the method presented. The computerized VTGN2 method facilitates work and saves time for the operator and provides instant test results. VTGN2 appears to be a sensitive indicator of peripheral bronchial obstruction, giving supplemental information to standard spirometry.