Past studies in nonclinical samples have found that suffocation fear, but not a behavioral index of carbon dioxide (CO2) sensitivity (i.e., breath-holding duration), predicts anxious response to CO2 challenge. These associations were examined in individuals with panic disorder while adding more sensitive indices of CO2 sensitivity. Consistent with the earlier studies, the authors found that suffocation fear predicted anxious responding to CO2 challenge but breath-holding duration did not. However, highly precise measures of CO2 sensitivity, not included in earlier studies, did predict anxious challenge responding. These findings support the predictive value and possible etiological relevance of both specific psychological variables and physiological CO2 sensitivity in panic vulnerability. Further work is still needed to determine whether the findings are specific to panic disorder.