The majority of health-related quality-of-life instruments are not preference-based measures and so cannot be used within cost utility analysis. The Asthma Quality of Life Questionnaire (AQLQ) is one such instrument. The aim of this study was to develop a health state classification that was amenable to valuation from the AQLQ, this being the 1st stage in developing a condition-specific preference-based measure for asthma patients. Rasch models were applied to samples of responders to the AQLQ with the aim of selecting a number of items for a preference-based utility measure (AQL-5D). Selection of items for the evaluation survey was supported with classical psychometric criteria for item selection (feasibility, internal consistency, floor and ceiling effects, and responsiveness). Rasch analysis proved to be a useful tool in assisting in the initial process of selecting items from an existing health-related quality-of-life instrument in the construction of the AQL-5D. The method is recommended for use alongside classical psychometric testing to aid in the development of preference-based measures.