OBJECTIVES: Multicentric Castleman’s Disease (MCD) is a rare lymphoproliferative disorder characterized by generalized lymphadenopathy and systemic symptoms such as fever, night sweats, fatigue, or loss of appetite. While many of these symptoms can best be assessed through patient-report there currently is no validated MCD Patient-Reported Outcome (PRO) instrument available to evaluate symptom severity and response to treatment. The purpose of this study was to develop an MCD Symptom Scale and assess its content validity through cognitive debriefing interviews. METHODS: A 16-item MCD PRO was developed through literature review, expert clinician input, and qualitative work with 12 patients diagnosed with MCD.  Subsequent cognitive debriefing was used to evaluate the content validity of the PRO with additional MCD patients (N=10) 6 male, 4 female, mean age 44 years. Study procedures were IRB approved, participants provided written informed consent, interviews were conducted using a discussion guide, audio-recordings were transcribed, and data were analyzed using qualitative analysis software (Atlas.ti). RESULTS: Emergent discussion of symptoms demonstrated that all MCD symptoms experienced by patients are evaluated in the PRO. Symptom experience was variable by patient, supporting the 24-hour recall period for capturing patient-reported symptom severity. All items were consistently understood by participants, who were able to select a response using a 5-point response scale to represent their symptom severity experience. CONCLUSIONS: The MCD Symptom Scale content was found to be comprehensive.  Design elements, including response options and recall period were suitable, and content validity was confirmed. The instrument was deemed acceptable for inclusion in clinical trials.  Psychometric testing is ongoing.