We examined lung specimens that fulfilled the pathological criteria of Bosken and were obtained from 5, patients with allergic bronchopulmonary aspergillosis/mycosis (ABPA/M). A finding common to all 5 was the presence of hard mucous plugs containing numerous eosinophils and fungal hyphae in the bronchi, showing central bronchiectasis. Bronchocentric granulmatosis with tissue eosinophilia (4 cases), xanthogranulomatous lesions (3 cases), eosinohilic pneumonia (2 cases) and organizing pneumonia (3 cases) were recognized only in bronchi distal to the mucous plugs (BMP). Almost normal lung parenchyma could be seen in some areas distal to the BMP. We found clusters of degenerated eosinophils (CDE) showing a "fir-tree like structure" and fungal hyphae in both the mucous plugs and the peripheral lesions. Most CDEs were free in the exudate and were not engulfed by macrophages. The presence of fungi in the mucous plugs may have caused marked inflammation in the wall around the BMP, which may have made the walls fragile and caused central bronchiectasis. The fungi in the mucous plugs may have caused the peripheral lesions via aerogenous dissemination. For this reason, we conclude that ABPA/M is an infectious disease, and that the primary lesion in ABPA/M is the formation of mucous plugs. Therefore, the presence of mucous plugs containing fungi and many eosinophils is diagnostic of ABPA/M.