Flexible fibre-optic bronchoscopy under local anaesthesia has been used to investigate the cellular airway events in atopic asthma. The findings have been compared to those from atopic individuals without asthma and non-atopic healthy controls, in an attempt to discern those changes relevant to clinical disease expression. Immunohistochemical and electron-microscopic analyses of airway biopsies identified that an atopic diathesis is associated with tissue eosinophil infiltration and mast cell degranulation. The eosinophilia was greatest in those atopic individuals with asthma. Flow-cytometric analysis of airway lavage revealed significantly enhanced T lymphocyte activation in clinical asthma. These findings are consistent with the hypothesis that T lymphocyte activation, through cytokine release, amplifies the tissue eosinophilia in asthma and that this combination is associated with clinical disease expression.