The main objective of this study is to investigate the ability of IOS to measure small airway disease by providing different impedance profiles for patients with asthma and COPD. Even though patients with asthma may show some reduction of the calibre of the small airways these changes are more a feature of patients with COPD. We hypothesize that the frequency dependent changes of IOS resistances will enable us to identify patients with COPD where we expect low frequency impedance to be higher compared to patients with asthma. Furthermore, in view of the increased lung compliance which characterizes COPD patients we expect reactance, an IOS measurement which reflects lung elastance, to be elevated compared to asthmatic subjects.