Summary The lungs of 12 patients suffering from ARDS treated by ECCO 2-R-therapy were examined. The thickness of the interlobular septa, area portions of honeycombing, bleeding and lung parenchyma with diffuse alveolar damage were assessed by semiautomatic image analysis. Medial thickness of the pulmonary arteries with an external radius ranging between 40 µm and 150 µm was assessed planimetrically. The area portion of honeycombing ranged from 0 % to 66 % (average: 28.8 %, standard deviation: 17.0 %). The thickness of interlobular septa was increased with values between 74.0 µm and 195.5 µm (average: 140.4 µm, standard deviation: 41.7 µm; normal controls: 72.0 µm ± 19.5 µm). Parenchymal and vascular changes were closely related: Average medial thickness rose from nearly normal values (4.9 %) in cases with low area portions of honeycombing and bleeding to the double (11.1 %) of normal values in cases with area portions of honeycombing and bleeding >40 %. We conclude that parenchymal changes due to shock and high inspiratory O 2 concentration lead to changes of the pulmonary arterial hemodynamics which result in a quickly developing medial hypertrophy.