Orthostatic intolerance affects an estimated 500,000 Americans and causes a wide range of disabilities. It is a disorder that more frequently affects young women (female-to-male ratio at least 4:1), often less than 35 years of age. Most of these patients experience an excessive heart rate increase when they stand. This heart rate increase is a sign that the cardiovascular system is working hard to maintain blood pressure and blood flow to the brain in the presence of a disordered cardiovascular regulation. Other than essential hypertension, OI is the most common disorder of blood pressure regulation. OI is also the most frequently encountered dysautonomia, accounting for the bulk of patients referred to centers specializing in autonomic disorders.