Capillary Leak Syndrome: A condition characterized by recurring episodes of fluid leaking from capillaries into extra-vascular compartments causing hematocrit to rise precipitously. If not treated, generalized vascular leak can lead to generalized EDEMA; SHOCK; cardiovascular collapse; and MULTIPLE ORGAN FAILURE.Capillary Permeability: The property of blood capillary ENDOTHELIUM that allows for the selective exchange of substances between the blood and surrounding tissues and through membranous barriers such as the BLOOD-AIR BARRIER; BLOOD-AQUEOUS BARRIER; BLOOD-BRAIN BARRIER; BLOOD-NERVE BARRIER; BLOOD-RETINAL BARRIER; and BLOOD-TESTIS BARRIER. Small lipid-soluble molecules such as carbon dioxide and oxygen move freely by diffusion. Water and water-soluble molecules cannot pass through the endothelial walls and are dependent on microscopic pores. These pores show narrow areas (TIGHT JUNCTIONS) which may limit large molecule movement.