OBJECTIVES: The responsiveness of the EQ-5D to patient-level changes over time in respiratory diseases appears limited. This study aimed to explore the potential room for adding a respiratory dimension to the current EQ-5D descriptive system, i.e. a bolt-on dimension. METHODS: First, the contents of the EQ-5D and six disease-specific health-related quality-of-life measures for asthma and COPD were reviewed. Second, a post-hoc analysis of data from three RCTs (N1=5452, N2=5992, N3=1086) in which both EQ-5D and disease-specific quality-of-life were measured, was conducted. Multivariate regression analyses were performed to investigate the change in the proportion of explained variance in EQ-5D VAS scores (R-square) when we added respiratory items of the disease-specific questionnaires to the EQ-5D dimension scores. Third, a principal component analysis (PCA) was performed to investigate the overlap between constructs of the disease-specific questionnaires and the EQ-5D. RESULTS: The review showed that all disease-specific questionnaires include questions about the impact of asthma/breathlessness on physical activities, while the EQ-5D addresses physical activities in terms of impact on mobility, self-care and usual activity.  Symptoms are addressed by five of the disease-specific questionnaires, while the EQ-5D covers them only broadly (pain/discomfort). Impact on functioning was included in four disease-specific questionnaires as well as the EQ-5D. Domains that were not included in the EQ-5D were impact on sleep and medication use. The regression analyses showed that the addition of questions about the impact of asthma/shortness of breath on physical activities led to the highest increase in R-square. In the PCA several disease-specific items were found that had no overlap with the EQ-5D. These constructs either related to symptoms or impact of breathlessness on activities. CONCLUSIONS: We conclude that symptoms and the impact of shortness of breath on physical activities are the most appropriate respiratory bolt-on candidates. A valuation study including both candidates is currently ongoing.