Chronic obstructive pulmonary disease (COPD) is a cytotoxic T lymphocyte (CD8)‐ and macrophage (CD68)‐predominant chronic inflammatory disorder of the conducting airways and alveoli. This is often associated with a neutrophilia, inflammation of small airways and destruction of tissue beyond the terminal bronchiolus, i.e. emphysema. In contrast, asthma is a helper T cell (CD4; type 2)‐predominant chronic inflammatory disorder of the conducting airways in which there is T lymphocyte‐derived gene expression for interleukin (IL)‐4 and IL‐5 but not interferon γ. There is fragility of airway surface epithelium, thickening of the reticular basement membrane, bronchial vessel congestion and (when severe) an increase in the mass of bronchial smooth muscle. This is usually (but not always) associated with tissue and peripheral blood eosinophilia rather than a neutrophilia and there is exudative plugging of the airways. These differences of inflammatory profile, remodelling and lung function are seen when smokers with COPD are compared with non‐smoking mild asthmatics. However there may be important similarities and overlap, particularly in more severe asthma when neutrophils predominate and in the older and/or smoking asthmatic when reversibility of airflow is less obvious. We have recently demonstrated gene expression for IL‐4 and IL‐5 in and around the mucus‐secreting glands of airways resected from smokers without a history of asthma. Also exacerbations of bronchitis may be associated with a tissue eosinophilia. On examination of bronchial biopsies from these patients we show surprisingly strong gene expression for IL‐4, IL‐5 and even human eotaxin and RANTES (regulated on activation normal T cell expressed and secreted). Whilst CD4 T lymphocytes of the Th2 phenotype might be expressing these cytokines in bronchitis, CD8 T lymphocytes are also capable of secreting IL‐4 and IL‐5. Viruses may modulate these changes in distinct lymphocyte functional phenotypes. The relevance and importance of CD4/CD8 T lymphocyte ratio to the development of COPD is discussed.