This report describes the ultrastructural findings in 37 patients who underwent open lung biopsy which yielded diagnoses of fibrosing alveolitis. A spectrum of lesions are categorized for the capillary endothelium and its basement membrane, the interstitial space and its fibrocellular components, and the alveolar epithelium and its basement membrane. The findings typify the different pulmonary cellular reactions to injury. Evidence for cellular regeneration and death in both epithelial and endothelial cell populations include atypical epithelial cell proliferation, capillary basement membrane multilamination, decrease in capillary lumen size and prominent pericytic ensheathment of pulmonary capillaries. Within the interstitium of the lung, proliferation of collagen and elastic fibers are documented, but in addition, abundant myofibroblasts and smooth muscle cells are present. No ultrastructural evidence of immune complex deposition was found in this study. The morphologic findings of fibrosing alveolitis further support the widespread concept that the lung responds to various injuries in a similar manner and undergoes a common reparative response regardless of etiology.