Variability in airway caliber is a characteristic feature of asthma. Previous studies reported that the variability in respiratory system impedance (Zrs), measured by the forced oscillation technique during several minutes of tidal breathing, is increased in asthma and may be a marker of inherent instability of the airways. The aims of this study were to determine if short-term variability in impedance correlates with peak expiratory flow (PEF) variability or airway hyperresponsiveness (AHR). The SD of log-transformed impedance (lnZrsSD) was measured as a marker of short-term variability and compared with the diurnal variability of PEF over 2 wk in 28 asthmatic and 7 nonasthmatic subjects and with AHR to histamine in a cohort of 17 asthmatic and 82 nonasthmatic subjects. In addition, lnZrsSD was measured in eight nonasthmatic subjects before and after methacholine challenge in the upright and supine positions. There were no significant differences in lnZrsSD between asthmatic and nonasthmatic subjects (P = 0.68). Furthermore, in asthmatic subjects, lnZrsSD did not correlate with diurnal variability of PEF (rs = -0.12 P = 0.54) or with AHR to histamine (r = 0.10, P = 0.71). Neither methacholine challenge nor supine posture caused any significant change in lnZrsSD. We conclude that our findings do not support previous reports about the utility of short-term variability of impedance. Our findings suggest that, using standard methods for forced oscillometry, impedance variability does not provide clinically useful information about the severity of asthma.