IntroductionCrohn’s disease and ulcerative colitis, the two major forms of inflammatory bowel disease (IBD), are chronic disabling conditions characterised by flares followed by periods of remission. However, patients with IBD are seen every 3–6 months in the outpatient clinic, and the occurrence of a flare between two outpatient visits is not captured. To our knowledge, there is no validated patient-reported outcome (PRO) tool to measure the phenomenon of flare in IBD. This study aimed to use an innovative methodology to collect messages posted by patients in an internet forum for developing and validating a PRO measuring flare in IBD.Methods and analysisThe design involves (1) computer engineering sciences for scraping extraction of messages posted in an internet forum and for identification of messages related to flare; (2) qualitative methods for thematic content analyse of the messages posted, for candidate items generation, for items selection (Delphi process) and for items adjustment (‘think-aloud’ interviews) and (3) quantitative methods for psychometric validation of the PRO.Ethics and disseminationEthical approval was obtained from the Comité de Protection des Personnes (CPP) CPP Nord-Ouest I (19.07.15.44139) in November 2019. The project aims to provide a tool to evaluate IBD flare in current medical practice that is constructed with patients’ perspectives. Items generation from a source corresponding to exchanges in an internet forum is an innovative method in this field and provides a wider coverage of qualitative data. If such a forum can result in interesting material, then this could be a new methodological perspective for generating items for questionnaires. Findings will be reported and disseminated widely through international peer-reviewed journal publications, oral and poster presentations at scientific conferences.Trial registration numberNCT04180345.