Acute interstitial pneumonitis (AIP) is a fulminant disease culminating in acute respiratory failure and often death.[ 1 ] Originally described in 1935 by the pathologists Hamman and Rich, this rare syndrome is characterized by rapidly progressive pulmonary fibrosis, leading to frequent confusion with idiopathic pulmonary fibrosis.[ 2 ] In fact, the eponym Hamman-Rich syndrome became synonymous with idiopathic pulmonary fibrosis despite clear differences in clinical presentation, radiography, pathology, and survival.[ 3 ] In 1986, Katzenstein described eight patients with acute respiratory failure of unknown etiology.[ 4 ] On biopsy, organizing diffuse alveolar damage was present in all specimens. Given the idiopathic nature of the disease, Katzenstein coined the phrase acute interstitial pneumonitis to distinguish it from the fibroproliferative stage of the acute respiratory distress syndrome (ARDS), which has an identical pathology.[ 5 ] Olson et al retrospectively examined Hamman and Rich's original cases, compared them to contemporary cases of AIP, and found the two identical.[ 6 ] Since then, little progress into understanding this disease has been made. Many questions still linger regarding the epidemiology, pathogenesis, and outcome. We recently published our experience with AIP providing new information regarding natural history.[ 7 ] This review summarizes the current literature on AIP emphasizing diagnostic criteria, pathogenesis, and natural history.