While portable spirometers are increasingly used, little attention has been paid to test their validity for measurement of flows in small airways. The aim of this study was to compare the Spirotel portable spirometer to a laboratory spirometer (Jeager PFT), with regard to accuracy in measuring forced expiratory flows, and more specifically those influenced by small airways (FEF(25-75)). Fifty-nine children (mean age, 12 years; range, 7-17), were studied at baseline and after a bronchodilator inhalation. Spirometers were tested separately in a randomly designed order. A total of 117 sessions of flow-volume curves was performed with each spirometer. We obtained at least two acceptable and reproducible curves in 88% and 76% of the sessions, with the laboratory and the portable spirometers, respectively. Unacceptable curves were easily detected by visual inspection of flow-time and flow-volume waveforms. Agreement was excellent between spirometers for the measurement of all expiratory flows, both at baseline and postbronchodilator. More specifically, agreement between spirometers was as high for measurements of FEF(25-75) (intraclass correlation coefficients 0.97) as for proximal flows. High correlations were found between baseline expiratory flows measured by each spirometer (and expressed as percent of predicted values), both in large and small airways (P < 0.001). The portable spirometer was highly sensitive for detecting small airways obstruction, as compared to the laboratory spirometer. Finally, the magnitudes of bronchodilator-related flow changes were also highly correlated, both in large and small airways (P < 0.001 and P = 0.004, respectively). We conclude that the Spirotel portable spirometer is reliable for measurement of forced expiratory flows, in large and small airways, provided that all curve waveforms can be stored and available for visual inspection.