A case of acute interstitial pneumonia with hypoxaemia is described; this occurred after the cessation of cortico steroids in a patient suffering from myeloma treated with melphalan. The absence of any microbes and the lymphocytosis in the bronchoalveolar lavage and the rapid and favourable improvement on cortico steroids led to a diagnosis of melphalan induced pneumonia. This acute form is probably due to a hypersensitivity mechanism and should be distinguished from the majority of cases of sub-acute fibrosing pneumonitis due to melphalan which have been published before. Urgent treatment with glucocorticoids is justified as well as the immediate and final cessation of the medication responsible, because it is this which will affect prognosis.