Rationale We hypothesize that severe pulmonary vascular remodelling in pulmonary arterial hypertension (PAH) is triggered by sustained endothelial injury in pre-capillaries caused by increased shear stress (SS). Therefore we tested whether human pulmonary microvascular endothelial cells (HPMVEC) from PAH patients are capable to adapt to high levels of SS. Methods and Results HPMVEC were isolated from PAH patients (autopsies and lung transplantations) and controls (rest material from lung tumor surgeries). The study has been approved by the IRB as non-WMO and consent was given. Static controls were compared to cells under physiological low (LSS, 2.3 dyn/cm2) and pathological high shear stress (HSS, up to 25 dyn/cm2). Different flow profiles (laminar, bifurcation-like) were applied and electrical resistance of the monolayers was recorded with ECIS (applied biophysics), as a measure for barrier function and cell behavior. Most striking are our findings in structural adaptation to HSS. Control HPMVEC re-aligned in flow direction, whereas HPMVEC from PAH patients failed to align after 72 hrs HSS challenge. Specifically, 65.3%±1.4 of the total number of control HPMVEC elongated, whereas 72.4%±4.4 of those cells re-aligned within an angle of less then 20 degrees relative to the flow direction (n=4, mean±SEM). Opposing only 47.2%±8.5 of PAH cells showed a stretched morphology and alignment randomly in the direction of flow (n=5, mean±SEM). Interestingly, upon HSS stimulation expression of the tested genes, known to play a role in shear sensing and adaptation (e.g. KLF2 and TGF-β), was up-regulated in both PAH and control samples. Under static culture conditions, PAH cells showed an irregular distribution and organization as well as decreased protein expression of the shear sensors VE-cadherin and CD31 (immunostaining and Western blot, n=4). Modeling of the electrical data gave further proof for weakened cell-cell adhesions in PAH cells. On the contrary, PAH and control HPMVEC formed similar endothelial barriers with a baseline resistance of 7767 vs. 8102 ohms. Here the ECIS modeling indicated an increased cell-matrix adhesion as possible cause. Furthermore, application of HSS to PAH HPMVEC caused severe cell loss, when bifurcation-like flow patterns were used. Control HPMVEC on the other hand were able to withstand those flow patterns. Conclusion and Relevance We showed that HPMVEC from PAH patients have a hampered ability to adapt, but not to sense pathological HSS, which can result in severe endothelial damage at vascular bifurcations. The causes for the failing adaptation are under testing and point towards a misbalance in cell-cell and cell-matrix adhesion. Our findings support the idea for a role of SS in the disease progression of PAH and imply, that new treatment strategies should consider normalization of the pulmonary blood flow to protect endothelial cells from injury and prevent vascular remodelling.