- Aim The scope of this recommendation is to provide guidance for reporting of inhibitor cases in previously treated patients (PTPs) with hemophilia A. This guidance is intended to improve transparency and completeness of reporting of observed events; it does not cover planning, executing or analyzing original studies aimed at the assessment of inhibitor rates. Recommendation We recommend that for each case of inhibitor development reported in a published paper, a paragraph or a table is included in the main publication reporting as a minimum the underlined data fields in Table . We recommend transparent reporting when any of the suggested information is not available. We recommend that particular care is used in reporting the timeline of events by clearly identifying a reference time-point. We suggest that journals in the field adopt this guidance as instructions for the authors and as a guide for reviewers. Conclusion Development of inhibitors in PTPs is a very rare event. Standardized reporting of inhibitor characteristics will contribute to generating a body of evidence otherwise not available. Case by case reporting of the recommended data elements may shed light on the natural history and risk factors of inhibitor development in PTPs and be useful for tailoring care in similar future cases.