Acute bronchitis/bronchiolitis (ABB) in the lung allograft is characterized by a predominantly neutrophilic infiltrate in the small and large airways and accompanied by other features such as luminal dilatation, mucous plugging, and granulation tissue formation. The etiologies for ABB are varied and depend on the context in which this lesion is found. Fifty-nine biopsies from 49 patients were found to have these changes. By correlating the clinical and histopathologic features we found ABB in one of five clinicopathologic categories: I) Harvest Injury (9 patients); II) Acute Cellular Rejection (7 patients); III) Bronchiolitis Obliterans Syndrome (14 patients); IV) Infection [prior to the development of bronchiolitis obliterans (OB)] (15 patients); and V) Other Manifestations of ABB (4 patients). In the context of early manifestations of harvest injury (Category I), ABB reflected severe ischemic lung injury with secondary acute inflammation of the airways. The prognosis was poor, with five patients dying and one requiring retransplantation because of irreversible harvest injury within 1 month of transplantation. When ABB was found in the setting of acute cellular rejection (Category II), it represented a severe manifestation of immunologic airway injury with a predominant lymphoplasmacytic response, and was followed by subsequent development of OB in five of seven patients. In those patients with histologically proven OB (Category III), the finding of ABB was present in a scarred or distorted airway and was a manifestation of airway rejection, infection, or both as demonstrated clinicopathologically, Infection-related ABB prior to the development of OB (Category IV) was managed as infection alone in 13 patients, but a coexistent perivascular lymphoplasmacytic infiltrate brought the concern for concurrent infection and rejection process in two patients. Since only two of the 15 patients in this category later developed OB, these patients with infectious ABB alone did not appear to be at a significant risk for the later development of OB. Finally, four patients demonstrated ABB without associated clinical manifestations and were placed in Category V (Other Manifestations of ABB). In this category, ABB was noted to be an indolent finding with all of the patients alive to date and none developing OB. Overall, the interpretation of ABB in the lung transplant setting depends on the recognition of the histologic clues and the clinical context in which one finds this airway lesion.