No single instrument can be used to identify asthma with certainty. Asthma is a clinical diagnosis made by physicians on the basis of a patient’s medical history, physical examination, assessment of the reversibility of airway obstruction, and exclusion of alternative diagnoses that mimic asthma. In epidemiologic studies, questionnaires are used to ascertain whether subjects have had symptoms of asthma or have ever received a diagnosis of asthma from a physician. The prevalence rates of both symptoms and diagnoses of asthma are heavily dependent on an awareness of asthma in the population studied. In recent decades, asthma has received widespread publicity, education of the public about asthma has increased, and new medications for asthma have been introduced. These measures may have increased public awareness of asthma and the rates of reporting of the disease.8,17,18,54 In Scotland,20 the proportion of children reporting asthma symptoms who received a diagnosis of asthma increased from 28% in 1964 to 64% in 1999, a result suggesting that part of the increase in the prevalence of asthma is attributable to changes in diagnostic labeling. The magnitude of the resulting bias in different populations is, however, difficult to appraise.