Acute interstitial pneumonia, first described by Hamman and Rich, is a rapidly deteriorating interstitial lung disease which even today still has a fatal course in at least 50%. Histologically, these patients are found to have the pattern of diffuse alveolar damage (DAD), characterised by hyaline membranes, alveolar oedema and a marked interstitial and alveolar inflammatory reaction. As a rule, the whole lung is uniformly affected. The changes cannot be differentiated from those found in the acute respiratory distress syndrome (ARDS). In the organisation phase, there is abundant type II cell hyperplasia and loose, fibrotic thickening of the alveolar septa. The disease is thought to be preceded clinically by a viral episode with myalgia, arthralgia, fever and malaise. The respiratory insufficiency develops relatively quickly. On initial contact, the picture of bilateral alveolar consolidation is often already present. Intubation with ventilation is more the rule and for many patients the ARDS criteria also apply. Recovery is possible but may also progress towards the terminal picture of honeycomb lung.