Phosphodiesterase type 4 plays an important role in modulating the activity of cells that are involved in the inflammatory processes occurring in chronic obstructive pulmonary disease and asthma. During the past decade, interest has focused on the development of selective inhibitors of phosphodiesterase-4, in an attempt to generate new treatment modalities that control airway inflammation not only in patients with asthma but also in those with chronic obstructive pulmonary disease. The aim of the present review is to discuss the results of recent clinical intervention studies with phosphodiesterase-4 inhibitors in patients with asthma or chronic obstructive pulmonary disease.