AbstractRecently, there has been much emphasis on the fact that there are many different phenotypes in asthma and chronic obstructive pulmonary disease (COPD). The aim of this review is to investigate some aspects of phenotyping in these two diseases.Epidemiological studies show a quite different risk factor pattern in allergic and non‐allergic asthma. Several studies also indicated that there are pathophysiological differences between these two types of asthma: such as eosinophil inflammation in allergic and neutrophil predominance in non‐allergic asthma. Neutrophil inflammation may also be a marker of severe asthma. At least one study indicates that non‐allergic asthmatics are less responsive to inhaled corticosteroids than allergic asthmatics. Recently, it has been emphasized that COPD also has manifestations other than lung function decline and that this also needs to be taken into account in severity characterisation. Dyspnea, weight loss, psychological status and co‐morbidity are factors that seem to be of large prognostic importance independently of the level of airflow obstruction in COPD.Progress has been made in understanding how asthma develops, but effective measures for primary preventions is still lacking. Better phenotyping in asthma will hopefully enable us to make progress in this area. Phenotyping is also important for assessment of prognosis in both asthma and COPD.Please cite this paper as: Janson C. Phenotypes of obstructive lung disease. The Clinical Respiratory Journal 2008; 2: 88–91.