Objective: To minimize the burden in detecting and monitoring Panic Disorder and Agoraphobia by developing a very brief scale with selected items from the Panic Disorder Severity Scale-Self Report (PDSS-SR), and to investigate the proposed scale's psychometric properties in a comorbid sample. Methods: A sample of 5103 patients from the Internet Psychiatry Clinic in Sweden, diagnosed and treated with Internet-based cognitive behavioral therapy for panic disorder (n = 1390), social anxiety disorder (n = 1313) or depression (n = 2400), responded to the PDSS-SR. Six criteria related to factor structure, sensitivity to change and clinical representativeness were used to select items. Psychometric analyses for the selected very brief scale were performed. Results: Items 2 (distress during panic attacks) and 4 (agoraphobic avoidance), were selected to create the very brief PDSS-SR version. Correlations with the full scale were high at screening, pre and post, and for change (0.87-0.93). Categorical Omega was omega(c) = 0.74. With a cut-off of 3 points, the scale could detect panic disorder in a psychiatric sample with a sensitivity of 85% and a specificity of 66%. Limitations: Limitations include lack of healthy controls and lack of blinding on secondary outcome measures. Conclusion: The proposed 2-item PDSS-SR version is a good candidate for a very brief panic disorder questionnaire, both for detecting cases and for measuring change. This is especially useful in clinical settings when measuring more than one condition at a time.