Asthma is a disease characterized by airway hyperresponsiveness (AHR). Methacholine (MCh) bronchoprovocation test is the most common method being used in clinics to detect AHR. However MCh-AHR has been found also in subjects without asthma. The aims of this study were to investigate the diagnostic value of MCh bronchoprovocation test and to characterize patients with suspected asthma. One hundred and eighty one patients with asthma-like symptoms who underwent the test were evaluated. Airway responsiveness was expressed as PC20 (mg/mL), the MCh concentration producing 20% fall in FEV1. One hundred and seven patients (59.1%) were diagnosed as having asthma, and about 2/3 of the remaining were allergic rhinitis and/or chronic sinusitis. Out of 107 asthma patients, 71 (66.4%), 85 (79.4%), and 89 (83.2%) showed PC20 <8 mg/mL, <16 mg/mL, and <25 mg/mL, respectively. At the cut off value of PC20 < 16 mg/mL, sensitivity, specificity, positive and negative predictive values of the test for asthma were 79.4%, 93.2%, 94.4%, and 75.8%, respectively. Asthmatics without MCh-AHR were diagnosed by alternative methods such as therapeutic response, histamine broncho-provocation test, exercise challenge test, diurnal variation of peak expiratory flow rate, bronchodilator test, and history of acute asthmatic attack. Asthma was found in 36 out of 110 patients (32.7%) with MCh-PC20 > or =8 mg/mL and in 22 out of 91 patients (24.2%) with PC20 > or =16 mg/mL. Most non-asthmatics with MCh-AHR had allergic rhinitis and chronic sinusitis. In conclusion, MCh bronchoprovocation test was confirmed to be a useful diagnostic method for asthma. However, because a negative MCh test could not completely rule out asthma in suspected patients, concurrent use of alternative methods may be warranted in diagnosing asthma.