|Oxidative stress has been implicated in a wide range of degenerative disorders from inflammatory disorders and cardiovascular disease to ageing and cancer (Halliwell 1999). Oxygen itself is a toxic agent, acting by increasing superoxide production through activation of xanthine oxidase and other oxidases. The antioxidant defences of the lung are of prime importance in protecting the body from oxidising agents such as the superoxide radical. Where the antioxidant defences of the lung are known to be impaired, free radical damage to lipids and proteins can occur. Cystic fibrosis, adult respiratory distress syndrome (ARDS) and asthma are such examples. This review will look at the extent of this defence system and the nature of the oxidative stress that causes it to break down in a variety of diseases.