Thirteen healthy subjects with infrequent panic attacks and without agoraphobia who did not meet DSM-III-R criteria for panic disorder, 43 patients with panic disorder, and 43 healthy control subjects who never experienced panic attacks underwent one vital capacity inhalation of 35% CO2. Healthy subjects with infrequent panic attacks reacted similarly to patients with panic disorder and more strongly than healthy subjects who never experienced panic attacks. The results suggest that (a) subjects with sporadic unexpected panic attacks and patients with panic disorder belong to the same spectrum of vulnerability and (b) CO2 hypersensitivity might be a trait marker of panic attacks rather than of a clinical diagnosis of panic disorder.