Desmoid tumors (or aggressive fibromatosis) are locally infiltrative connective-tissue tumors that can arise in any anatomic location; they can be asymptomatic, or they can result in pain, deformity, swelling, and loss of mobility and/or threaten visceral organs with bowel perforation, hydronephrosis, neurovascular damage, and other complications. Existing clinical trial endpoints such as the Response Evaluation Criteria in Solid Tumors (version 1.1) and progression-free survival are inadequate in capturing treatment efficacy. This study was designed to develop a novel clinical trial endpoint by capturing patient-reported outcomes (PROs). Following best practices in qualitative methodology, this study used concept elicitation (CE) interviews to explore desmoid patients' perspectives on key disease-related symptoms and impacts. Qualitative analysis was performed to determine the relative frequency and disturbance of symptoms and impacts as well as other characteristics of these concepts. A draft PRO scale was then developed and tested with cognitive interviewing. Information from the interviews was subsequently incorporated into the refined PRO scale. CE interviews with desmoid patients (n = 31) helped to identify salient concepts and led to a draft scale that included symptom and impact scales. Cognitive interviews were completed with additional patients (n = 15) across 3 phases. Patient input was used to refine instructions, revise and/or remove items, and modify the response scale. This resulted in an 11-item symptom scale and a 17-item impact scale. This is the first disease-specific PRO instrument developed for desmoid tumors. The instrument is available as an exploratory endpoint in clinical trials. This study highlights the feasibility and challenges of developing PRO instruments for rare diseases. © 2019 American Cancer Society.