OBJECTIVES: Patient-reported outcome (PRO) measures provide relevant information on how patients function and feel about their health. Such data are useful in clinical practice and registrational trials. Despite the importance of symptom assessment in CDAD, there is no validated PRO for Clostridium difficile-associated diarrhoea (CDAD). A qualitative research study was conducted to develop a CDAD PRO according to US FDA PRO guidelines. METHODS: Content development comprised 2 study phases, with input from an advisory group of 6 CDAD experts in Europe and North America. Phase I elicited patients' experiences of CDAD symptoms in open-ended discussions during telephone interviews. Supplementary interviews obtained nurses' observations. A draft PRO was developed following demonstration of concept saturation. Readability and translatability were assessed. Phase II involved 2 rounds of patient interviews, with revision of the draft PRO after each round. All patients were ≥18 years old, with confirmed CDAD. IRB approval and participant informed consent were obtained. RESULTS: Phase I interviews included 18 patients and 6 nurses in the United States; 16 additional patients were interviewed in Phase II. Patients were representative of the general CDAD population, and diverse in age, gender, and disease severity. Concept saturation was reached in Phase I for spontaneously reported CDAD symptoms. Items were organised in a draft conceptual framework with 5 hypothesised domains: diarrhoea, abdominal discomfort, tiredness, lightheadedness, and other symptoms. Phase II demonstrated initial content validity of the 13-item draft daily diary (CDAD-DaySyms™). Participants reported the questions were clear, relevant, and comprehensive; were able to use the instructions to complete the diary correctly; and considered the 24-hour recall period appropriate. CONCLUSIONS: The CDAD-DaySyms™ captures symptoms relevant to CDAD patients, demonstrating initial content validity. To allow its use in clinical practice and CDAD clinical studies, final content and psychometric validity are being evaluated in 2 ongoing international clinical trials.