INTRODUCTION: Patients with eosinophilic gastritis and/or gastroenteritis (EG/EGE) experience significantly reduced health-related quality of life related to the symptoms and signs of their disease. There is a need to develop a PRO questionnaire for clinical research in EG/EGE. The objectives of this study were to develop a symptom questionnaire (drafted using literature and clinician input to determine preliminary measurement concepts) and then (1) identify, describe, and substantiate signs, symptoms, and impacts of EG/EGE from the perspective of adults with the condition via concept elicitation methods; and (2) evaluate the properties of the questionnaire and make modifications via cognitive debriefing methods. METHODS: Interviews were conducted, transcribed, coded, and analyzed. Eligible adults with EG/EGE first participated in a concept elicitation segment where they described their experiences with EG/EGE, continuing until saturation was achieved, adding symptoms to the draft symptom questionnaire as needed. This was followed by a cognitive debrief to assess the appropriateness of the draft symptom questionnaire, where patients assessed readability, comprehensibility, relevance, and comprehensiveness. The questionnaire was modified based on patient feedback, then re-evaluated to confirm its appropriateness. RESULTS: In total, 16 interviews were conducted and 24 signs and symptoms were reported during concept elicitation. The most frequently reported (Figure 1) were abdominal pain, nausea, diarrhea, vomiting, and abdominal cramping; while abdominal pain, nausea, and diarrhea were the most bothersome to patients. Saturation was achieved after 12 interviews. In the cognitive debriefing, revisions included the addition of a vomiting frequency item and revision of the early satiety item for clarity. Response options were modified to instruct the participants to assess each symptom “at its worst”. For the final version of the EG/EGE-SQ©, participants were able to read, comprehend, and provide appropriate responses, and reported that the resulting questionnaire was comprehensive for their condition. CONCLUSION: The results of this research support the content validity, including comprehensiveness and interpretability, of the EG/EGE-SQ© which offers the potential for use in clinical research to assess symptoms in patients with EG/EGE. Future research is needed to assess its psychometric properties including reliability and validity, responsiveness, and potential for use in pediatric populations.