A comparison was made of total respiratory resistance (Rrs) and reactance (Xrs) determined between 6 and 26 Hz by means of a forced oscillation technique in 27 patients with asthma (group A), 28 patients with chronic bronchitis (group B), and 20 patients with emphysema (group E) to examine whether the method can provide data capable of distinguishing among these diseases. The three groups demonstrated a similar reduction in the FEV1. In addition, static and dynamic lung volumes, maximal expiratory flows, diffusing capacity for CO, airway resistance (Raw), and elastic lung recoil were measured. The observed alterations of Rrs and Xrs consisted of an increase in Rrs, a decrease in Rrs with frequency, and a decrease in Xrs. However, significant differences were found between the three groups: Rrs at 6 Hz and average Rrs were most increased in group A, whereas the negative frequency dependence of Rrs and the decrease in Xrs were least pronounced in group E. In all groups Rrs at 6 Hz, the average slope of Rrs, and the average level of Xrs were tightly interrelated and showed in addition a high correlation with airway resistance; the correlations with FEV1 were less satisfactory. Discriminant analysis performed on the complete set of data, excluding diffusing capacity and elastic lung recoil, which were used as selection criteria, demonstrated that forced oscillation parameters were the most important factors discriminating among the groups.