A series of autonomic function tests were performed in eight patients who had orthostatic intolerance and orthostatic tachycardia (> 36 bpm). All eight had an abnormal tilt-table test (mean tachycardia 50 bpm compared with 22 +/- 7 bpm in 31 normal subjects). Plasma volume was low in four out of six patients. There was a subnormal response to the cold pressor test in three out of eight patients. Intravenous atropine-induced tachycardia was subnormal in two out of eight patients. A segmental or patchy anhidrosis was present in six out of eight patients. Pilocarpine (0.0625%) induced miosis (three out of six) and exaggerated Valsalva ratio (one out of eight) were observed. A follow-up of six patients, 8-17 years after the autonomic evaluation, showed spontaneous and complete improvement in two out of six, partial improvement in one out of six, and persistence of symptoms in three out of six patients. A repeat autonomic evaluation in two patients with persistent symptoms, 2 and 5 years after the initial assessment, showed mild progression of sympathetic and parasympathetic dysfunction. This syndrome appears characterized by orthostatic tachycardia, low plasma volume, lack of adrenergic supersensitivity, and sudomotor abnormalities. A longitudinal follow up demonstrates patients with and without recovery. The syndrome appears heterogeneous in terms of onset, clinical manifestations, laboratory studies and outcome.