CONTEXT: Chronic or refractory breathlessness is common and distressing. To evaluate new treatments, outcome measures that capture change in patients' experience are needed. OBJECTIVES: To explore the extent to which the numerical rating scale (NRS) worst and average, and the Chronic Respiratory Questionnaire capture change in patients' experience during a trial of mirtazapine for refractory breathlessness. METHODS: Convergent mixed-methods design embedded within a randomized trial comprising 1) semi-structured qualitative interviews (considered to be the gold standard) and 2) outcome measure data collected pre- and post-intervention. Data were integrated, exploring examples where findings agreed and disagreed. Adults with advanced cancer, chronic obstructive pulmonary disease, interstitial lung disease, or chronic heart failure, with a modified Medical Research Council dyspnea scale grade 3 or 4 were recruited from three U.K. sites. RESULTS: Data were collected for 22 participants. Eleven had a diagnosis of chronic obstructive pulmonary disease, eight interstitial lung disease, two chronic heart failure, and one cancer. Median age was 71 (56-84) years. Sixteen participants were men. Changes in the qualitative data were commonly captured in the NRS (worst and average) and the Chronic Respiratory Questionnaire. The NRS worst captured change most frequently. Improvement in the emotional domain was associated with physical changes, improved confidence, and control. CONCLUSION: This study found that the NRS using the question "How bad has your breathlessness felt at its worst over the past 24 hours?" captured change across multiple domains, and therefore may be an appropriate primary outcome measure in trials in this population. Future work should confirm the construct validity of this question.