The forced oscillation technique (FOT) is a noninvasive test used to characterize the mechanical impedance of the respiratory system. The aim of the study was to compare the changes in respiratory conductance (Grs) measured with FOT to those in FEV1 in 22 patients with asthma and 20 patients with chronic obstructive pulmonary diseases (COPD) after salbutamol inhalation. FEV1 and Grs indexes, computed as the ratio of the difference between postbronchodilator and prebronchodilator values over the predicted value, were used to express reversibility of airway obstruction. After inhalation of salbutamol in cumulative doses up to 1,200 micrograms in ten patients of each group, FEV1 and Grs indexes showed parallel changes, and most of the increase was observed after the first dose of 200 micrograms of salbutamol for the two indexes. In all the 42 patients, we found a linear relationship between the two indexes after inhalation of 200 micrograms of salbutamol (r = 0.7, p < 0.0001). We evaluated FEV1 and Grs indexes in terms of sensitivity and specificity for identifying asthmatics among patients with COPD: using a 10% change as the cut-off value, these indexes proved of similar value (sensitivity, 0.91 and 0.95; specificity, 0.95 and 0.85, respectively). We conclude that the use of FOT can be considered as an alternative to forced expiration for detecting bronchodilatation in asthmatics and patients with COPD.