Panic disorder is characterized by recurrent anxiety attacks in the absence of a frightening stimulus1. It is a common disorder, affecting 2-5 of the general population and 10-14 of patients seen in cardiology practice 2,3. Infusion of sodium (DL)lactate precipitates an anxiety attack in most persons with this disorder but rarely does so in normal controls, suggesting a neurobiological basis for the problem4-6. Despite this observation, the patho-physiology of panic disorder remains unknown. We have now used positron emission tomography to measure cerebral blood flow (CBF) in patients with panic disorder in the absence of a panic attack. Analysis of CBF in regions thought to mediate symptoms of panic, anxiety and vigilance reveals a significant (P< 0.005) abnormal asymmetry of CBF (left<right) located in a region of the parahippocampal gyrus. This asymmetry was present in seven patients with panic disorder and a positive response to lactate infusion but was absent in six normal controls and in three patients with panic disorder associated with a negative response to lactate. We believe this to be the first study to identify a discrete brain abnormality in patients with this severe form of anxiety.