Purpose: For patients with celiac disease (CeD), the only current management option is adherence to a strict gluten-free diet (GFD); however, many patients on a GFD continue to experience symptoms with a significant impact on quality of life. Potential new treatments for CeD are under development and a validated patient-reported outcome measure is required to evaluate their utility in clinical trials. The purpose of this article is to provide a history of the development of the Celiac Disease Symptom Diary (CDSD) 2.1 © for use in clinical trials., Methods: Qualitative and quantitative studies were conducted from 2010 to 2021, including concept elicitation and cognitive debriefing interviews with adult and adolescent participants with CeD (N = 93) diagnosed via biopsy and/or serology and input from eight interviews with CeD clinical experts. During these studies, different iterations of the CDSD were presented to the US Food and Drug Administration and the European Medicines Agency, and modifications were made in line with their feedback., Results: These studies ultimately led to the development of CDSD 2.1 © , a daily diary which focuses on key symptoms of CeD (abdominal pain, bloating, diarrhea, nausea and tiredness). This patient-reported outcome measure was readily understood by adult and adolescent participants with CeD and content validity was demonstrated in both populations., Conclusion: CDSD 2.1 © is a content-valid patient-reported outcome measure developed in accordance with best practices and regulatory guidance. A thorough exploration of the psychometric properties of CDSD 2.1 © for both adult and adolescent participants with CeD is ongoing to support utilization in clinical trials., Competing Interests: Declarations. Conflict of interest: KH is an employee of IQVIA, Montreal, Quebec, Canada and was an employee of ICON plc during the development of CDSD 1.0©. DA currently serves on the faculty of the University of California, San Francisco, CA, USA and was an employee of Alvine Pharmaceuticals at the time of the development of CDSD version 1.0©. SG is an employee of Takeda Development Center Americas Inc., Cambridge, MA, USA and receives stock or stock options. SA is an owner and shareholder of Acaster Lloyd Consulting Ltd, London, UK and was an employee of Oxford Outcomes Ltd (later acquired by ICON plc) during the development of CDSD 1.0©. SC is an employee of Sprout Health Solutions, Los Angeles, CA, USA and was an employee of ICON plc during the development of CDSD 1.0©. CPK is an employee of Beth Israel Deaconess Medical Center, Boston, MA, USA and has acted as a consultant to COUR Pharmaceuticals, Janssen, Kanyos/Anokion, Merck, Milky Way Life Sciences, Takeda Pharmaceuticals and Theravance and has received stock options from COUR Pharmaceuticals. SAM is an employee of RTI Health Solutions, Ann Arbor, MI, USA. RTI Health Solutions were paid consultants to conduct the qualitative research evaluating the CDSD v2.1©. LMM was an employee of Takeda Development Center Americas Inc., Cambridge, MA, USA during development of CDSD 2.1© and holds Takeda stock. DAL is an employee of Beth Israel Deaconess Medical Center, Boston, MA, USA and Takeda Development Center Americas Inc., Cambridge, MA, USA and receives stock or stock options. Ethics approval: The studies described in this manuscript were approved by the relevant Institutional Review Board (IRB) for each institution (see Online Resource). Consent to participate: Written IRB-approved informed consent documents (or assent documents for participants 17 years of age or younger) were obtained from all participants prior to taking part in each study. Each study was fully explained to all potential participants, including the purpose of the study, the type of questions asked and the time required. Participants were able to ask questions before agreeing to participate., (© 2024. The Author(s).)