The Panic Disorder Severity Scale (PDSS) [Shear et al., 1997] is rapidly gaining world-wide acceptance as a standard global severity measure of panic disorder, however, its cross-cultural validity and reliability have not been reported yet. We developed the Japanese version of the PDSS and examined its factor structure, internal consistency and inter-rater reliability and concurrent validity among Japanese patients with panic disorder with or without agoraphobia. We also established rules of thumb for interpreting PDSS total scores, taking the Clinical Global Impression severity scale as the anchoring criterion. The identical one-factor structure of the PDSS was confirmed among the Japanese patients as among the United States patients. Both internal and inter-rater reliability was excellent (Cronbach's alpha was 0.86, and ANOVA ICCs were all above 0.90). Concurrent validity of the PDSS items with self-report questionnaires tapping similar or overlapping domains was satisfactory (Pearson correlation coefficients were mostly above 0.5). Using the anchor-based approach, the following interpretative guides are suggested: among those with established panic disorder diagnosis, PDSS total scores up to 10 correspond with "mild," those between 11 and 15 with "moderate," and those at or above 16 correspond with "severe" panic disorder. The present findings support the cross-cultural generalizability of panic disorder symptomatology and of the PDSS, in particular.