Specimen arteriography, morphometry, and light and electron microscopy were used for examination of the pulmonary vasculature of 22 patients who died with the adult respiratory distress syndrome (ARDS), for the purpose of defining the lesions that contribute to pulmonary hypertension in this setting. The different lesions correlated with the duration rather than the cause of ARDS. Thromboemboli occurred in 21 patients, and macrothrombi found at autopsy correlated with the number of filling defects on antemortem angiography. Acute endothelial injury was documented ultrastructurally even in intermediate and late-stage patients. Fibrocellular intimal obliteration of arteries, veins, and lymphatics and infective vasculitis were prominent in those surviving beyond 10 days. In long-term survivors, tortuous arteries and irregularly dilated capillaries were striking features. Peripheral extension of vascular smooth muscle and a significant increase in the percentage of medial thickness of muscular arteries with duration of ARDS were noted. The pathogenesis and clinical significance of these lesions is discussed.