Allergic rhinitis (AR) is a risk factor for developing clinical asthma. Moreover, AR is often associated with bronchial hyper-responsiveness (BHR). The aim of this study was to compare the degree of involvement of the peripheral airways during metacholine (MCh) challenge test in asthmatics and patients with AR with or without BHR by using the impulse oscillometry (IOS) technique. Fifty-three patients with seasonal AR were investigated with MCh challenge test and IOS. Thirteen healthy non-reactive subjects served as controls. MCh challenge test was performed, cut-off value FEV1PD20 2000 microg. Linear regression analysis was used to calculate a MCh/FEV1 slope (Slope-FEV1MCh), which was used as an index of BHR. IOS was performed, measuring respiratory resistance and reactance during the MCh challenge test. Twenty-six subjects had both AR and asthma symptoms, 27 patients reported AR only, without asthma symptoms, 17 (63%) manifested BHR by spirometric criteria. Patients with AR and symptoms of asthma manifested greater peripheral airway obstruction compared to those with AR only with BHR. Increased peripheral obstruction in the asthmatic group compared to patients with AR and BHR may explain previous findings that asthmatics perceive greater airway obstruction during MCh challenge tests.