During the last 15 years the use of sputum induction by inhalation of hypertonic saline solution to study the cellular and biochemical composition of the airways has been increased significantly. The induced sputum technique, which is inexpensive and does not require complex instruments, allows a relatively non-invasive sampling of airway content. Thus, adequate samples of lower airway secretions can be collected even from patients who are not able to produce sputum spontaneously and the features of airway inflammation in asthma, chronic obstructive pulmonary disease (COPD) and other respiratory diseases can be investigated. The method of induced sputum induction has been shown to be reproducible, sensitive and valid.