Acute interstitial pneumonia (AIP) is the pathologic correlate of Hamman-Rich syndrome, a rapidly progressive interstitial fibrosis of unknown etiology. The onset of shortness of breath is typically preceded by a prodrome of viral-like illness with cough and fever, and respiratory failure within days or weeks. AIP has many pathologic features of organizing diffuse alveolar damage (see Chapters 71 and 87). There is no known precipitating factor for AIP, as opposed to acute respiratory distress syndrome (ARDS), which almost always follows an acute injury or trauma. Mortality is high, up to 60%. Cultures for viruses, bacteria, and fungal organisms are negative in the early stages; however, nosocomial infections may occur during hospitalization.