BACKGROUND: Many patients with asthma-like symptoms show upper airway hyperresponsiveness (UHR). However, few investigators have studied the etiologies of UHR. OBJECTIVE: To investigate the etiologies of UHR. Method: During methacholine bronchoprovocation tests, acceptable inspiratory flow volume curves were obtained. UHR was defined as methacholine concentration causing a 25% fall in maximal mid-inspiratory flow (PC(25)MIF(50)) <25 mg/mL and upper airway obstruction (UAO) as maximal mid-expiratory/inspiratory flow ratio >1.0. RESULT: The PC(25)MIF(50) was significantly related to bronchial hyperresponsiveness index (r=0.68, P<0.001), blood eosinophil counts (r=-0.31, P<0.01) and serum total IgE level (r=-0.24, P<0.05). More than half of the patients with UHR/UAO had concurrent allergic rhinitis and the abnormality in paranasal sinus X-rays. Sixty-six (64.1%) of the 103 patients with UHR or UAO were female. The prevalence of smoking, blood eosinophilia, increased IgE levels and positive skin tests was significantly lower in women than in men. Women were significantly older than men (P<0.01) and had a higher prevalence of UAO (P<0.05). CONCLUSION: UHR or UAO seems to be most closely associated with allergy and bronchial hyperresponsiveness. However, women had a higher prevalence of UAO which may be related to non-allergic factors such as a smaller airway caliber.