Organizing pneumonia, or bronchiolitis obliterans organizing pneumonia, may result from any of multiple potential causes in lung transplant patients, including causes that may be seen in the general population (see Chapter 62). Organizing pneumonia may result from infections, including those from bacterial, viral (CMV, HSV, RSV), and fungal organisms (Candida, Aspergillus, Pneumocystis); or from aspiration pneumonia. Organizing pneumonia can be seen associated with acute cellular rejection, resolving acute cellular rejection, or chronic rejection; or in association with other acute lung injuries such as ischemia-reperfusion injury. Organizing pneumonia has also been reported as a complication of the immunosuppressive drug sirolimus.