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Methods of classifying chronic obstructive pulmonary disease (COPD) depend largely upon spirometric measurements but disability is only weakly related to measurements of lung function. With the increased use of pulmonary rehabilitation, a need has been identified for a simple and standardised method of categorising disability in COPD. This study examined the validity of the Medical Research Council (MRC) dyspnoea scale for this purpose.One hundred patients with COPD were recruited from an outpatient pulmonary rehabilitation programme. Assessments included the MRC dyspnoea scale, spirometric tests, blood gas tensions, a shuttle walking test, and Borg scores for perceived breathlessness before and after exercise. Health status was assessed using the St George's Respiratory Questionnaire (SGRQ) and Chronic Respiratory Questionnaire (CRQ). The Nottingham Extended Activities of Daily Living (EADL) score and Hospital Anxiety and Depression (HAD) score were also measured.Of the patients studied, 32 were classified as having MRC grade 3 dyspnoea, 34 MRC grade 4 dyspnoea, and 34 MRC grade 5 dyspnoea. Patients with MRC grades 1 and 2 dyspnoea were not included in the study. There was a significant association between MRC grade and shuttle distance, SGRQ and CRQ scores, mood state and EADL. Forced expiratory volume in one second (FEV1) was not associated with MRC grade. Multiple logistic regression showed that the determinants of disability appeared to vary with the level of disability. Between MRC grades 3 and 4 the significant covariates were exercise performance, SGRQ and depression score, whilst between grades 4 and 5 exercise performance and age were the major determinants.The MRC dyspnoea scale is a simple and valid method of categorising patients with COPD in terms of their disability that could be used to complement FEV1 in the classification of COPD severity.