Postmortem and biopsy lung samples were examined with a dissecting microscope and classified into three categories: no overdistended air spaces visible, mild destruction and some fenestration of the septa with minimal confluence of air spaces, and more advanced breakdown of alveolar walls with at least a sixfold increase in air space to a 1300 mu diameter. Prominent ultrastructural tissue alterations were found within the focal regions where fenestration of the alveolar wall and some confluence of air spaces were identified. Edema and rearrangement of the supportive tissues were noted in the alveolar septa adjacent to foci having the greatest alveolar enlargement. Randomized electron dense collagen fibrils were found only in these areas, whereas the normal alveolar septa contained uniformly parallel collagen fibrils. Many of the randomized fibrils had sufficient swelling to display prominent internal spiraling. Other changes in structure and cell numbers were consistently found in association with the altered collagen. These included: separation of the type I epithelial cells from the basal lamina and some autolysis of the cytoplasmic organelles, numerous flattened type II epithelial cells, and aggregations of pulmonary macrophages.