The overall aim of the present investigation was to examine the association between the subscales of the ASI and emotional responding to voluntary hyperventilation challenge in a panic disorder population. Based on findings from [J. Abnorm. Psychol. 110 (2001) 372.], we predicted that the AS-Physical Concerns subscale would best predict the fear response to hyperventilation. We also examined the relative contribution of each of the three ASI subscales in predicting behavioral tolerance to hyperventilation. Participants (N = 192) meeting DSM-IV criteria for panic disorder with or without agoraphobia completed the Anxiety Sensitivity Index (ASI) and underwent a voluntary hyperventilation challenge. Consistent with prediction, the AS-Physical subscale significantly predicted subjective fear during the hyperventilation challenge (12% of variance accounted for); whereas only the AS-Social subscale accounted for significant variance (4%) in patients' behavioral tolerance to the hyperventilation challenge.