The two obstructive airway diseases bronchial asthma and chronic obstructive pulmonary disease (COPD) represent major global causes of disability and death, and COPD is estimated to become the third most common cause of death by 2020. The structural and pathophysiologic findings in both diseases appear to be easily differentiated in the extremes of clinical presentation. However, a significant overlap may exist in individual patients regarding features such as airway wall thickening on computer tomography or reversibility and airway hyperresponsiveness in lung function tests. Airway inflammation differs between the two diseases. In bronchial asthma, airway inflammation is characterized in most cases by an increased number of activated T-lymphocytes, particularly CD4+ Th2 cells, and sometimes eosinophils and mast cells. The most notable difference of chronic severe asthma compared with mild to moderate asthma is an increased number of neutrophils. In stable COPD, airway inflammation is characterized by an increased number of T-lymphocytes, particularly CD8+ T cells, macrophages and neutrophils. With the progression of the disease severity, macrophage and neutrophil numbers increase. Although there may be a partial overlap between asthma and COPD in some patients, the differences in functional, structural and pharmacological features clearly demonstrate the consensus that asthma and COPD are different diseases along all their stages of severity.