Diffuse panbronchiolitis (DPB) is a clinicopathologic entity characterized histologically by chronic inflammation principally affecting the respiratory bronchioles. Few pathologists outside Japan are familiar with this entity. The most distinctive pathologic feature of DPB is chronic inflammation and an accumulation of foam cells in the walls of the respiratory bronchioles, adjacent alveolar ducts, and alveoli (PB unit lesion). The differential diagnosis is important both clinically and histologically because of the similarity of DPB to other chronic airway diseases. We report seven cases of DPB and 20 examples of a histologically similar lesion (PB-like lesion) found in a large review of cases of bronchiolitis, cystic fibrosis, bronchiectasis, aspiration pneumonia, extrinsic allergic alveolitis. Wegener's granulomatosis, bronchocentric granulomatosis, and malignant lymphoma. The results indicate that the PB-like lesion is a nonspecific histologic finding; the diagnosis of DPB can be made only in the appropriate clinical setting and when other conditions have been carefully ruled out.