The idiopathic interstitial pneumonias have unknown etiology and are characterized by diffuse parenchymal lung involvement and the potential to develop pulmonary fibrosis. Most portend a reduction in life expectancy due, in part, to the absence of effective therapies. The symptoms of idiopathic interstitial pneumonia develop insidiously. In contradistinction, acute interstitial pneumonia (AIP, also known as acute interstitial pneumonitis) is unique in that it has a very rapid to fulminant onset, leading to early hospitalization and a high initial case fatality ratio but, potentially, a more favorable long-term prognosis for survivors. Despite its contemporary description nearly 20 years ago, knowledge of this disease has increased little. This review focuses on AIP and its current place among the idiopathic interstitial pneumonias.