This study will determine if airway microorganisms are associated with lung function deficits and non-allergic inflammation in childhood asthma. Importantly, these characteristics may be the underlying basis of non-response to conventional therapy in many asthmatics. We will recruit 240 children; 80 asthmatics with irreversible airflow obstruction, 80 with “normal” or reversible lung function, and 80 non-asthmatics. Clinical status will be assessed using physiological and immunological tests. Induced sputum will be examined for airway microorganisms and inflammation. We will assess whether specific bacteria are associated with asthma characteristics (particularly persistent airflow obstruction). Interactions between airway microorganisms, severity, symptoms and inflammation will also be examined. On the basis of this, microorganisms associated with persistent airflow limitation will be identified. This will guide the development of tailored interventions (e.g. the use of antibiotics) to preserve or improve lung function in childhood asthma thereby preventing more severe asthma and COPD in later life.