Allergic bronchopulmonary aspergillosis (ABPA) occurs in patients with asthma and cystic fibrosis when Aspergillus fumigatus spores are inhaled and grow in bronchial mucus as hyphae. Chronic colonization of Aspergillus fumigatus and host's genetically determined immunological response lead to ABPA. In most cases, lung biopsy is not necessary because the diagnosis is made on clinical, serologic, and roentgenographic findings. Some patients who have had lung biopsies or partial resections for atelectasis or infiltrates will have histologic diagnoses. A number of different histologic diagnoses can be found even in the same patient. In the early stages the bronchial wall is infiltrated with mononuclear cells and eosinophils. Mucoid impaction and eosinophilic pneumonia are seen subsequently. This may be followed by bronchiolitis obliterans, granulomatous bronchiolitis, and pulmonary fibrosis. Treatment with corticosteroids appears to prevent the progression of the disease.