Limitation of activity and impaired quality of life are important outcomes of COPD. There is an association between measures of self-reported physical activity and overall health status, and they appear to change together spontaneously over time and in response to treatment. The relationship between symptoms and activity limitation is complex, because activity can be limited entirely by symptoms, or impaired by symptoms so that it requires greater effort or causes discomfort. The patient has the choice of whether to restrict their activity or maintain it at the cost of having symptoms. In theory, this may make it difficult to produce reliable standardized assessments of activity limitation because it may not be clear exactly what is being measured. Analysis of items in the St George's Respiratory Questionnaire (SGRQ) concerned with activities that are either not possible due to breathlessness, or are a cause of breathlessness, show that they contribute to a unidimensional model of activity limitation in daily life and a unidimensional model of overall COPD-related health status. The items lie distributed evenly along the same severity continuum, from very mild to very severe, along with other items concerned with symptoms and the psycho-social impact of the disease. This suggests that self-reported limitation of activity may form a reliable construct, and may also provide a good surrogate marker of health status in COPD.