Orthostatic intolerance (OI) is the development of symptoms when standing upright that are relieved when reclining. There are many types of orthostatic intolerance. OI can be a subcategory of dysautonomia, a disorder of the autonomic nervous system occurring when an individual stands up. Some animal species with orthostatic hypotension have evolved to cope with orthostatic disturbances.

A substantial overlap is seen between syndromes of orthostatic intolerance on the one hand, and either chronic fatigue syndrome or fibromyalgia on the other. It affects more women than men (female-to-male ratio is at least 4:1), usually under the age of 35.

Orthostatic intolerance occurs in humans because standing upright is a fundamental stressor, so requires rapid and effective circulatory and neurologic compensations to maintain blood pressure, cerebral blood flow, and consciousness. When a human stands, about 750 ml of thoracic blood are abruptly translocated downward. People who have OI lack the basic mechanisms to compensate for this deficit. Changes in heart rate, blood pressure, and cerebral blood flow that produce OI may be caused by abnormalities in the interactions between blood volume control, the cardiovascular system, the nervous system, and circulation control system.