Total and differential cell counts from hypertonic‐induced, dithiothreitol‐dispersed sputum provide reproducible measurements of airway inflammatory cell counts, which are responsive to treatment with anti‐inflammatory drugs. They have helped to understand the kinetics of inflammatory cell changes in asthma after the reduction of corticosteroids and the subsequent re‐introduction of treatment. They have identified that the presence of sputum eosinophilia in asthma, chronic cough and chronic airflow limitation is a predictor of steroid‐responsiveness and of lack of ‘asthma control’. They can be used to study the dose–response effect of inhaled corticosteroids and may be useful to establish the relative potency of different corticosteroid formulations and delivery devices. Sputum cell counts are also useful to study the potential anti‐inflammatory effects of drugs like theophylline, long‐acting β‐adrenoceptor agonists, leukotriene antagonists and newer drugs in development. They may be helpful to select add‐on therapy to corticosteroids in ‘difficult‐to‐control’ asthma.