We investigated the basis on which respiratory physicians establish a diagnosis of asthma in clinical practice. A questionnaire survey was conducted on physicians in charge of 1,600 asthma patients receiving outpatient treatment at the Hokkaido University General Hospital or its affiliated hospitals or clinics. The bases for diagnosis were ranked as follows: 1) recurrent paroxysmal dyspnea or symptoms such as wheezing and cough (86%); 2) detection of wheezing on auscultation (78%); 3) improvement in symptoms or auscultation findings after using bronchodilators (56%); 4) improvement in symptoms or auscultation findings after using inhaled corticosteroids (55%); 5) diagnosis of asthma by another physician (46%); and 6) spirometry findings (31%). The performance rates of each examination were as follows. Spirometry at initial visit, 47%; sputum eosinophils, 25%; improvements in FEV1 measured after inhalation of bronchodilator or after treatment as asthma, 12%; measurements of airway responsiveness, 5% and variability in peak expiratory flow, 2%. Asthma is often diagnosed by respiratory physicians based on symptoms, physical examinations and improvement in symptoms or physical findings after treatment for asthma in medical practice. The performance rates of tests used to diagnose asthma were low.