The single-frequency forced oscillation technique was used to estimate the total respiratory impedance, resistance, and reactance at 2, 4, and 12 Hz of children who had cricoid stenosis (CS; n = 11), provoked bronchoconstriction (PB; n = 6), or cystic fibrosis (CF; n = 13). The selection criteria for patients who had CS and CF were abnormal forced expiratory volume in 1 sec (FEV!) and/or maximal expiratory flow at 50% vital capacity (Vmax 50%). Sixteen of the 17 patients who had CS or PB were found to have resistance (Rrs) values outside the normal range at 4 Hz; at 2 Hz, 14 were found to have abnormal values and, at 12 Hz, only 9. The reactance in the CS and PB groups was somewhat less discriminative than Rrs at all frequencies. Of the 13 patients who had CF, only 4 had abnormal Rrs values; in this clinical condition, the reactance at 2 Hz was the most discriminative variable, being abnormal in 6 of the 13 patients. Irrespective of the clinical group, the Rrs was, in absolute terms, highest at 2 Hz, and it decreased with increasing frequency. This pattern of negative frequency dependence was, however, not obviously specific for any of the tested clinical conditions and offered no information in addition to a single low-frequency determination.