We describe the pathologic features in surgically excised lung tissue specimens from 18 cases of allergic bronchopulmonary aspergillosis (ABPA). The main abnormalities involved bronchi and bronchioles. All cases showed bronchocentric granulomatosis (BCG), mucoid impaction of bronchi (MIB), or both. The impacted mucin of MIB contained large numbers of eosinophils and Charcot-Leyden crystals. A distinctive exudative bronchiolitis was present distal to areas of BCG in 13 cases. This lesion was characterized by filling of bronchiolar lumens with necrotic neutrophils and eosinophils in a basophilic mucinous exudate. A peribronchiolar chronic inflammatory infiltrate was seen in 15 cases; eosinophils were prominent in 10 of these cases. Foci of eosinophilic pneumonia were seen in 13 cases, and noninvasive fungal hyphae were identified in 14. We conclude that the finding of BCG or MIB, or a combination of both, especially in conjunction with tissue eosinophilia, should suggest the diagnosis of ABPA. When noninvasive fungal hyphae are also present, the changes are diagnostic of ABPA or a related allergic fungal reaction.