Methotrexate, an antifolate cytotoxic drug, is used in anticancer chemotherapy as well as an immuno suppressive in rheumatoid arthritis. It is responsible for numerous secondary effects, amongst which is a characteristic acute pneumonia known since 1969. This pneumonitis has been described in detail, up to the present time in 78 cases gathered in this review. The prevalence of this complication is estimated at around 7%. This pneumonia may occur whatever the age, indication for which methotrexate is prescribed, the route of administration of the product (including the intra-thecal route) and the dose. It includes dyspnoea, fever, (sometimes quite marked) and frequently an acute reversible respiratory failure. Radiologically the opacities are usually diffuse interstitial and symmetrical with a basal predominance with sometimes some confluence and occasionally a pleural reaction. In a small number of cases a transient mediastinal adenopathy has been described. Respiratory function tests show a rapidly developing restrictive syndrome accompanied by hypoxia and hypocapnia. Broncho-alveolar lavage is characterised by hypercellularity with a frank and apparently transitory lymphocytosis. Histologically the most frequent lesion sighted is an extensive acute granulomatous reaction with or without oedema. Most often the outcome is favourable (75% of cases). However 6 deaths due to respiratory failure have been reported. Even though there has not been any formal test, steroid therapy in high dosage seems to accelerate recovery. Progress to an irreversible pulmonary fibrosis is possible but rare. The mechanism of this drug related acute pneumonia is not known but would seem to resemble that of other granulomatosis. Besides this rapidly progressive pneumonitis, methotrexate is responsible for a very small number of cases of severe pulmonary oedema and of acute painful pleurisies.