Increased blood-brain-barrier (BBB) permeability precedes any clinical or pathologic signs and is critical in the pathogenesis of multiple sclerosis (MS) and brain metastases. CD4+ TH1 cells mediate demyelination in MS, but how they get sensitized and enter the brain to induce brain inflammation remains obscure. TH2 cytokines associated with allergic disorders have recently been implicated in MS, while genes upregulated in MS plaques include the mast cell-specific tryptase, the IgE receptor (Fc-epsilon-RI) and the histamine-1 receptor. Mast cell specific tryptase is elevated in the CSF of MS patients, induces microvascular leakage and stimulates protease-activated receptors (PAR), leading to widespread inflammation. BBB permeability, MS and brain metastases appear to worsen in response to acute stress that leads to the local release of corticotropin-releasing hormone (CRH), which activates brain mast cells to selectively release IL-6, IL-8 and vascular endothelial growth factor (VEGF). Acute stress increases BBB permeability that is dependent on CRH and mast cells. Acute stress shortens the time of onset of experimental alleric encephalomyelitis (EAE) that does not develop in W/W mast cell deficient or CRH -/- mice. Brain mast cell inhibition and CRHR antagonists offer novel therapeutic possibilities.