Lung morphology in ARDS reflects the rapid evolution from interstitial and alveolar edema to end-stage fibrosis consequent to injury of the alveolocapillary unit. This morphologic progression, termed diffuse alveolar damage, has been subdivided into sequentially occurring exudative, proliferative, and fibrotic phases. Pulmonary lesions correlate with the phase of alveolar damage rather than with its specific cause. As suggested by experimental models of acute lung injury, the pathologic features seen in humans are consistent with the effects of a host of injurious stimuli and the complex interaction of inflammatory mediators on alveolar epithelial and capillary endothelial cells. Although ARDS frequently culminates in "interstitial" fibrosis, the organization of intraluminal exudate dominates the histologic picture in the proliferative phase and establishes the framework for subsequent fibrous remodeling of the lung. Involvement of the pulmonary vasculature is an important aspect of ARDS, from the initial phase of edema to the terminal stage of intractable pulmonary hypertension. Vascular lesions include thrombotic, fibroproliferative, and obliterative changes, which, like the parenchymal lesions, correlate with the temporal phase of diffuse alveolar damage. Although ARDS is characterized by extensive bilateral lung involvement, alveolar damage can affect the lung in a localized fashion. "Regional alveolar damage" is associated with the same clinical risk factors as diffuse damage, suggesting that there is a spectrum in the extent of lung involvement and disease severity in patients at risk for ARDS. The factors that govern which patients will develop the fulminant syndrome are poorly understood. It must be re-emphasized that the lung is stereotyped in its response to injury, and, consequently, descriptive, or even quantitative, studies of lung morphology can only provide clues regarding the initiating factors and pathogenetic mechanisms of ARDS. Progress in understanding the pathogenesis of ARDS and the devising of rational approaches to therapy will ultimately depend on careful clinical and experimental studies that unravel basic mechanisms of cellular injury and response. The course of these investigations must be guided by and constantly correlated with the pathologic features that occur in humans.