Trends in the prevalence of symptoms suggestive of asthma show greater variation than trends in the prevalence of diagnosed asthma (Fig. 1B). The prevalence of asthma symptoms increased in most countries up to the 1990s, but since then there has been no clear temporal pattern. The variability among countries may in part be attributable to differences in definitions of asthma symptoms, such as wheezing, shortness of breath, and asthma attacks. Recent data from the International Study of Asthma and Allergies in Childhood (ISAAC) phase 3 showed that in the mid- 1990s, the prevalence of wheezing increased in most centers where the prevalence had been low and decreased or remained unchanged in most centers where the prevalence had been high.