Experimental evidences suggest that Panic Disorder (PD) is characterized by abnormalities in respiratory and vestibular functions. We studied balance system function in patients with PD and its relationships with CO(2) reactivity and clinical characteristics. Nineteen patients with PD with/without agoraphobia underwent static posturography and the 35% CO(2) challenge. The severity of clinical symptomatology was measured by standardized psychometric scales. Patients were free of psychotropic medications during the 2 weeks before the study. Different investigators blind to each other carried out the CO(2) challenge, static posturography and clinical assessment. Nineteen age and sex-matched healthy controls underwent static posturography. Body sway velocity and length were significantly higher in panic patients than in controls and patients showed high percentages of abnormal scores. Patients with two or more abnormal scores on static posturography were significantly more agoraphobic than those with less than two. Abnormal posturography scores under the eyes-opened was related to high anticipatory anxiety, whereas those under eyes-closed was related to phobic avoidance. Symptomatological reactivity to CO(2) was significantly correlated to abnormal functions of the balance system in the eyes-closed condition. Our findings suggest that (1) many patients with PD (5-42%) have abnormalities in their balance system function compared with healthy controls (0-5%), (2) symptomatological reactivity to CO(2) and balance system function in patients with PD are correlated only in the eyes-closed condition and (3) there is a significant link between agoraphobic avoidance and subclinical abnormal function of the balance system network.