How is the severity of chronic asthma best assessed and what are the mechanisms that determine its severity? These two questions are difficult to answer because, until we understand what asthma is, it is not possible to describe its severity in a given individual. In the shorter Oxford English Dictionary severity in relation to disease is defined as “violence or acuteness of illness” or “grievousness of affliction”. The former definition is useful for exacerbations or attacks. Clinicians have had little difficulty in agreeing on defining the severity of attacks.1 ,2 The latter definition seems relevant to the severity of the chronic disease which is the subject of these comments. Until recently the severity of asthma was rarely assessed, but with the increasing use of guidelines and drug trials the concept of severity has emerged. The GINA guidelines1classify the disease as intermittent and persistent with severe, moderate, and mild degrees of the persistent disease. There are now a number of scores available to define the severity of asthma3-5 based on a combination of symptoms, measurements of airway function, and need for medication.