The organizing phase of diffuse alveolar damage (DAD) generally begins approximately 7 days after an acute lung injury, and is characterized by dramatic proliferation of interstitial fibroblasts associated with marked hyperplasia of type 2 pneumocytes, and distortion of lung architecture. Organized DAD may grossly show firm smooth areas of consolidated lung parenchyma and small cysts. Organized DAD must be distinguished from other forms of fibrosis, including usual interstitial pneumonia (UIP). In the early stages of organization of DAD, the fibrosis is predominantly immature. In rare cases, the fibrosis may progress to mature fibrosis and result in honeycomb change. Whereas UIP has a slow progressive course, DAD has a rapid onset, often arising in the setting of severe inflammatory or traumatic events (see Chapter 71).