To analyze microvascular changes, angiograms and microscopic sections of the lung were prepared from 12 patients dying after severe adult respiratory distress syndrome (ARDS) of diverse cause. Anatomic features of the precapillary arterial bed were morphometrically assessed and related to the duration of acute lung injury. In intra-acinar arteries, medial thickness was increased after short duration ARDS (less than 12 days). Preacinar arteries developed medial thickening only after ARDS of long duration (18–60 days). Mean external diameter (ED) of partially muscular intra-acinar arteries was less than normal in both duration groups, whereas ED of muscular arteries was less in the long duration group. Concentrations of partially and fully muscular arteries tended to be higher in ARDS lungs. These results give evidence of an extension of smooth muscle into normally nonmuscular, smaller pulmonary arteries. Total artery concentration in 7 of the 12 patients was less than the normal, and dilatation of intra-acinar muscular arteries was often observed. The ARDS is characterized by partial or complete disruption and even disappearance of much of the pulmonary vascular bed.