- This study attempted to replicate previous reports of associations between panic disorder and joint hyperlaxity. The authors also examined possible associations between reacitivity to carbon dioxide (CO 2), a model for panic vulnerability, and hyperlaxity in healthy volunteers. One hundred and one patients with DSM-IV panic disorder and 39 healthy volunteers were assessed for hyperlaxity by Beighton's criteria. Healthy volunteers also received two vital capacity inhalations of CO 2. Thirteen (13%) patients had ®ve or more hyperlax joints. This rate did not differ from that in the healthy volunteers. Anxiety in healthy volunteers , as measured by the NIMH self-rating scale, DSM-IV panic symptom scores, and 100 mm visual analog scales of anxiety, increased after CO 2 from a mean of 1.8 to 2.8 (not signi®cant); from 0.5 to 4 (p ` 0.001) and from 8.7 to 11.6 mm (p ` 0.1), respectively. There were no associations between responses to CO 2 and hyperlaxity.