Background: The Panic Disorder Severity Scale (PDSS) is promising to be a standard global rating scale for panic disorder. In order for a clinical scale to be useful, we need a guideline for interpreting its scores and their changes, and for defining clinical change points such as response and remission. Methods: We used individual patient data from two large randomized controlled trials of panic disorder (total n 5 568). Study participants were administered the PDSS and the Clinical Global Impression (CGI)-Severity and -Improvement. We applied equipercentile linking technique to draw correspondences between PDSS and CGI-Severity, numeric changes in PDSS and CGI-Improvement, and percent changes in PDSS and CGI-Improvement. Results: The interpretation of the PDSS total score differed according to the presence or absence of agoraphobia. When the patients were not agoraphobic, score ranges 0-1 corresponded with ''Normal,'' 2-5 with ''Borderline,'' 6-9 with ''Slightly ill,'' 10-13 with ''Moderately ill,'' and 14 and above with ''Markedly ill.'' When the patients were agoraphobic, score ranges 3-7 meant ''Borderline ill,'' 8-10 ''Slightly ill,'' 11-15 ''Moderately ill,'' and 16 and above ''Markedly ill.'' The relationship between PDSS change and CGI-Improvement was more linear when measured as percentile change than as numeric changes, and was indistinguishable for those with or without agoraphobia. The decrease by 75-100% was considered ''Very much improved,'' that by 40-74% ''Much improved,'' and that by 10-39% ''Minimally improved.'' Conclusion: We propose that ''remission'' of panic disorder be defined by PDSS scores of five or less and its ''response'' by 40% or greater reduction. Depression and Anxiety 26:922-929, 2009.