Classical radiation pneumonitis has been described after single dose whole lung irradiation in experimental animals where above a threshold dose of irradiation, there is a sigmoid dose response curve with increasing morbidity and mortality. After clinical fractionated irradiation, however, acute radiation pneumonitis consisting of cough shortness of breath and patchy radiological changes, occurs in <10% of patients, has dyspnoea out of proportion to the volume of lung irradiated and usually resolves completely without long-term effects. There is increasing evidence that this represents a bilateral lymphocytic alveolitis or hypersensitivity pneumonitis and has been termed sporadic pneumonitis. Late radiation toxicity results in pulmonary fibrosis. This is a consequence of repair, which is initiated by tissue injury within the radiation portal. It follows release of chemotactic factors for fibroblasts including transforming growth factor-beta, fibronectin and platelet derived growth factor. Radiation fibrosis is the clinically more significant syndrome for patients. It may result in progressive dyspnoea and mortality in patients. The most predictable change in laboratory lung function tests is a decrease in transfer factor due to damage at the capillary-alveolar level. It also results in decreased lung compliance, which will affect the total lung capacity and the forced vital capacity. The forced expiratory volume in 1 s is less affected, although this seems to depend on the volume of lung irradiated. There is also a decrease in perfusion in the irradiated lung. Radiation fibrosis seems to depend, amongst other factors, on the volume of lung, which is irradiated above a threshold of 20-30 Gy. The morbidity of radiation fibrosis may therefore be minimized by the use of dose volume histogram to minimize the volume of normal lung irradiated in patients at high risk, e.g., patients with who present with poor lung function. The importance of the baseline perfusion in the irradiated areas continues to be studied.