Tumor necrosis following photodynamic therapy appears to be due primarily to effects on the vasculature. The endothelium is probably the main target for these effects. Damage to endothelial cells and their endocapillary layer may lead to release of clotting factors and vasoactive compounds, resulting in vaso-constriction, thrombus formation and platelet aggregation. Trapped red blood cells (RBC) are then exposed to light for sufficiently long time to start the photohemolytic process. The swollen RBC fill the luminal space, causing complete blockage of the vessel. Later on, after lysis of RBC, the vessel walls may collapse because of high extravascular pressure. Permanent shut off of the circulation in tumor tissue cause hypoxia, nutrient deprivation and eventual tumor cell death.