Asthma is characterised by a persistent, ongoing airway submucosal inflammatory process which may underlie bronchial hyperresponsiveness and airways obstruction. Indirect assessment of airway inflammation includes examination of sputum or serum for eosinophils or eosinophil-derived products, but examination of bronchial mucosal biopsies or bronchoalveolar lavage fluid or cells remains the gold standard. Lymphocyte and eosinophil activation, with the release of certain cytokines such as interleukin-5, lipid mediators such as leukotrienes and eosinophil products such as major basic protein, may be important mechanisms of inflammation in asthma. Less invasive indirect measures of submucosal inflammation are needed to assess the impact of asthma treatment in the clinic. Antiinflammatory treatment in asthma should be introduced as early as possible. A greater understanding of the inflammatory mechanism or of the actions of corticosteroids may lead to the development of more effective and safer drugs for asthma.