This study was designed to compare spirometers used for human testing and to determine whether the results obtained by different spirometers meeting the American Thoracic Society (ATS) requirements are interchangeable. Water-sealed spirometer (Harvard), dry bellow wedge spirometer (Vitalograph) and computerized pneumota-chograph (Gould), all of them satisfying the ATS recommendations were compared. Forced vital capacity (FVC), forced expiratory volume in the first second (FEV1), forced expired flow between 25 and 75% of FVC (FEF25-75%) and 100 FEV1/FVC(FEV1%) were selected for comparative analysis. Measurements of these parameters were carried out on a total of 40 healthy volunteers of mixed nationalities. The Vitalograph values for FVC, FEV1 and FEV1% were significantly higher than those of the water-sealed spirometer (Harvard), but were closely similar to the values obtained by the Gould computerized pneumotachograph. Our results thus do not support the interchangeability of different spirometers and stress the importance of biological standardization of spirometers against each other.