Bronchiolar emphysema is a rare, insidious, progressive, bilateral pulmonary fibrosis with diffusion defect. Death follows in five to 10 years after the onset of symptoms because of respiratory failure, cor pulmonale or pneumothorax. In the 20 cases reviewed, the lungs typically showed bosselation of their pleural surfaces, giving the appearance and consistency of a liver involved by Laennec's cirrhosis. Cross sections showed honeycombing chiefly along the lung margins. Microscopically, cystic dilatation of terminal bronchioles was present, with obliteration of the remaining peripheral lung tissue by a dense pulmonary fibrosis. Hyperplasia of smooth muscle and elastica was prominent.Bronchiolar emphysema is a clinicopathologic entity, the etiology of which is unknown and likely multiple. Bronchiolectasis and superimposed recurrent infection are essential in its pathogenesis. Thickening of alveolar walls may play a decisive role in determining the characteristic site of the bronchiolar dilatation.