Introduction <p>Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare, autosomal dominant disorder characterized by episodic yet cumulative heterotopic ossification (HO) of connective tissues. Flare-ups, i.e., sites of local soft tissue inflammation, are associated with swelling, joint stiffness, and pain are commonly associated with FOP. This study aimed to investigate the content validity of the Flares Diary by gaining an in-depth understanding of the experience of FOP flare-ups in people with FOP and by debriefing the Flares Diary to assess the relevance, comprehensiveness, and understandability of the instrument.</p> Methods <p>Adults with FOP who participated in the Phase 2 LUMINA-1 trial (NCT03188666) of garetosmab, or who were recruited by email via the International Fibrodysplasia Ossificans Progressiva Association, were enrolled in this study. Trained qualitative researchers conducted in-depth semi-structured interviews with all participants comprising concept elicitation and cognitive debriefing of the Flares Diary, with participants encouraged to describe flare-up symptoms and impacts in their own words. All interviews were conducted between March 3, 2022, and March&#xa0;7, 2023. A conceptual model was developed based on participant-reported symptoms and impacts of flare-ups.</p> Results <p>Thematically analyzed interview transcripts from 20 adults with FOP identified concepts of localized and systemic symptoms, as well as impacts on participants’ daily lives associated with flare-ups. Minimal issues were reported with understanding the Flares Diary, and participants found that the items were comprehensive and matched their own experience of flare-ups.</p> Conclusion <p>The results confirm the adequacy of the Flares Diary and that the symptoms assessed are important to people with FOP.</p> <p>Graphical abstract available for this article.</p> Trial Registration <p>ClinicalTrials.gov identifier, NCT03188666.</p> Graphical Abstract <p></p>