OBJECTIVE: Autonomic dysregulation and cerebral blood flow (CBF) abnormalities have been reported in patients with anxiety disorders and, more recently, in panic disorder. Variations in the middle cerebral artery velocity (measured by transcranial Doppler technique), heart rate, and blood pressure during a tilting-table test were used as nonanxiogenic procedures to explore these abnormalities. METHOD: Mean flow velocity in the right middle cerebral artery, heart rate, and blood pressure were monitored at rest and during a 70 degrees tilting- table test. Three groups of patients were studied: 11 patients (eight women and three men; mean age = 31.1 years, SD = 8.0) with a diagnosis of panic disorder within 6 months of the onset of the disorder, nine asymptomatic patients (two men and seven women; mean age = 35.0, SD = 6.3) with a previous diagnosis of panic disorder who were in full remission of symptoms and had been drug free for at least 6 months, and 10 normal comparison subjects (two men and eight women; mean age = 31.1, SD = 5.2). RESULTS: Both patients with acute panic disorder and patients with remitted panic disorder showed a significant percent reduction of right middle cerebral artery mean flow velocity compared with normal subjects following tilting to the upright position. No significant differences were observed for blood pressure and heart rate. CONCLUSIONS: Patients with panic disorder, both during the acute phase of the illness and after clinical recovery, show an exaggerated drop in CBF during tilting. Since these findings are similar, although attenuated in intensity, to those which are observed in dysautonomic illnesses, one possible interpretation is that of panic disorder as a subclinical form of autonomic dysreactivity.