Anxiety disorder patients (n = 198; under criteria of the Diagnostic and Statistical Manual of Mental Disorders; rev. 3rd ed.; American Psychiatric Association, 1987) and nonanxious control subjects (n = 25) underwent challenges of 90 s of voluntary hyperventilation and 15 min of 5.5% carbon dioxide in air. Panic disorder subjects showed a greater subjective response to both challenges than did subjects with other anxiety disorders, who in turn responded more than did control subjects. Furthermore, subjects with panic disorder as an additional diagnosis tended to report more subjective response than did anxiety disorder subjects without panic disorder. The best prechallenge predictor of response to each procedure was a measure of fear of physical symptoms. The findings support previous results that have pointed to a greater fear or anxiety-inducing effect of these challenge procedures in panic disorder patients, as compared with other subjects.