PURPOSE: The modified-Medical Research Council (mMRC) breathlessness scale consists of five grades that contain of a description of different activities. It has wide utility in the assessment of disability due to breathlessness but was originally developed before the advent of modern psychometric methodology and, for example contains more than one activity per grade. We conducted an evaluation of the mMRC structure. PATIENTS AND METHODS: Cognitive debriefing was conducted with COPD patients to elicit their understanding of each mMRC activity. In a cross-sectional study, patients completed the mMRC scale (grades 0-4) and an MRC-Expanded (MRC-Ex) version consisting of 10-items, each containing one mMRC activity. Each activity was then given a 4-point response scale (0 "not at all" to 4 "all of the time") and all 10 items were given to 203 patients to complete Rasch analysis and assess the pattern of MRC item severity and its hierarchical structure. RESULTS: Cognitive debriefing with 36 patients suggested ambiguity with the term "strenuous exercise" and perceived severity differences between mMRC activities. 203 patients completed the mMRC-Ex. Strenuous exercise was located third on the ascending severity scale. Rasch identified the mildest term was "walking up a slight hill" (logit -2.76) and "too breathless to leave the house" was the most severe (logit 3.42). CONCLUSION: This analysis showed that items that were combined into a single mMRC grade may be widely separated in terms of perceived severity when assessed individually. This suggests that mMRC grades as a measure of individual disability related to breathlessness contain significant ambiguity due to the combination of activities of different degrees of perceived severity into a single grade.