OBJECTIVES: To demonstrate how patient interviews reinforce a conceptual model of endometriosis symptoms and impact and help to identify concepts for inclusion in an improved Endometriosis Pain Daily Diary (EPDD). METHODS: Concept elicitation (CE) interviews were conducted with 17 surgically diagnosed endometriosis patients in the US to identify the PRO measurement concepts related to symptoms and impacts of endometriosis-related pain most important and relevant to patients’ experience of endometriosis.  Items were developed based on the Endometriosis Pain and Bleeding Diary (EPBD) and existing conceptual data. Cognitive interviews (CI) were conducted with 14 patients in the US to confirm comprehension of the draft items and ability to respond to the items as intended. RESULTS: Based on both spontaneous mentions and the frequency with which concepts were expressed during the CE interviews, dysmenorrhea and non-menstrual pain were the predominant symptom concepts.  Other commonly reported symptoms were specific pelvic pain both during and between periods, deep dyspareunia, dyschezia, cramping, back pain, heavy bleeding and nausea.  The most predominant impact concepts were difficulty performing both daily and work activities.  Results support a preliminary conceptual model based on literature review and expert interviews. Draft instrument items were tested and refined through the CI process, and the draft EPDD was finalized for subsequent psychometric testing in preparation for use in clinical trials. CONCLUSIONS: The FDA PRO Guidance states that measures should be conceptually valid as they relate to the disease being studied, meet a threshold of psychometric soundness, and be relevant to patients. This research represents an important step toward establishing the EPDD as “fit for purpose” relative to the FDA PRO Guidance for use in support of label claims; and supports the development of a diary that better reflects the American Society for Reproductive Medicine’s recommendations for outcome measures in pain clinical trials.