The aim of this study was to verify whether the increase of resistance to airflow using a filter at the mouth could determine significant systematic errors of measurement or change the diagnostic performance in a respiratory physiology laboratory. The effect of the new filter Spirobac (DAR Spa, Italy) was assessed on out-patients referred to our laboratory for routine functional evaluation. The following tests were performed: maximal expiratory flow-volume curve, plethysmographic lung volume and airway resistance, bronchodilator test, and methacholine challenge test. Each test was performed randomly, twice with the filter and twice without. Significant differences between measurements with and without filter were found for forced vital capacity, forced expiratory volume in one second, airway resistance, and specific airway conductance (sGaw). These differences were unrelated to the average values of the measurements, except for sGaw. The limits of agreement were within the range of intraindividual short-term repeatability for almost all of the function indices. The overall concordance between tests performed with and without filter was 78% for bronchodilator test and 53% for methacholine test. However, in all the cases but one, the concordance of the methacholine test was inside the short-term repeatability. We conclude that the filter Spirobac has a statistically significant effect on the results of some pulmonary function tests. However, this is not considered to be clinically significant, since the changes due to the filter were within the intraindividual variability of repeated measurements for almost all the function indices and no appreciable classification error was found in diagnostic tests.