Excessive erythrocytosis results in severely increased blood viscosity that may have significant detrimental effects on endothelial cells and ultimately function of the vascular endothelium. Since blood brain barrier stability is crucial for normal physiological function, we investigated the effect of excessive erythrocytosis on vessel number, structure and integrity in vivo using our previously characterised Epo-overexpressing transgenic mouse line (termed tg6) that has a hematocrit of 0.8-0.9. These mice have abnormally high nitric oxide (NO) levels, a potent pro-inflammatory molecule, suggesting altered vascular permeability and function. In this study we observed that brain vessel density of tg6 mice was significantly reduced (16%) and vessel diameter significantly increased (15%) compared to wt. Although no significant increases in vascular permeability under normoxic or acute hypoxic conditions (8% O₂, 4h) were detected, electron microscopy analysis revealed that tg6 endothelium had altered morphological characteristics. Tg6 brain vascular endothelial cells appeared to be activated with increased luminal protrusions reminiscent of ongoing inflammatory processes. In agreement with this observation we detected increased levels of ICAM-1 and vWF, markers of endothelial activation and damage, in brain tissue. We propose that chronic excessive erythrocytosis and sustained high hematocrit causes endothelial damage that may ultimately increase susceptibility to vascular disease.