The goal of the FLUPRO study was to develop a patient reported outcome (PRO) measure to assess symptoms of influenza (FLU) and influenza-like illness for use in clinical studies and to define disease severity including a valid measurement scale of patients’ symptoms. The study included three stages: Stage I. Elicitation interviews including item phrasing, length, response options, recall. The outcome of Stage 1 was a draft instrument and item pool suitable for evaluation at Stage II. Stage II. Evaluative interviews to assess comprehension and interpretability of the written questionnaire from the respondent´s perspective, adjusting the items as needed. The outcome of Stage II was a final instrument and item pool ready for quantitative empirical evaluation Stage III. Item reduction and instrument validation to evaluate item properties, reduce the number of items, and validate the performance of the final measure quantitatively.