The lungs of 12 patients suffering from ARDS treated by ECCO2-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 semi-automatic image analysis. Medial thickness of the pulmonary arteries with an external radius ranging between 40 microns and 150 microns 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 microns and 195.5 microns (average: 140.4 microns, standard deviation: 41.7 microns; normal controls: 72.0 microns +/- 19.5 microns). 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 greater than 40%. We conclude that parenchymal changes due to shock and high inspiratory O2 concentration lead to changes of the pulmonary arterial hemodynamics which result in a quickly developing medial hypertrophy.