In orthostatic intolerance, the patient develops symptoms while standing that are relieved when the patient assumes a supine position. Different degrees of orthostatic intolerance exist, but not a system of grading severity. We have developed a system that grades the severity of orthostatic intolerance by the three-pronged criteria of the rapidity of development and the severity of orthostatic symptoms, the ability of the subject to withstand orthostatic stresses, and the degree of interference with daily living. In this article, this system is presented, and one disorder, postural tachycardia syndrome (POTS), is examined in some detail.