Objectives. The impact of disease on functioning is the essential information for clinicians when reporting on health problems of individuals. The International Classification of Functioning, Disability and Health (ICF) is a comprehensive and universally accepted model to classify and describe functioning, disability and health in a systematic way. The objective of this article is to outline the development and validation of a health index for patients with AS based on the ICF as a use case. Methods. The project is a combined effort of the Assessment of SpondyloArthritis International Society, the ICF Research Branch of the World Health Organization (WHO) Collaboration Centre of the Family of International Classifications and the WHO. There are five steps in the development and validation of the health index for patients with AS: (i) development of an item pool; (ii) identification of candidate items; (iii) item selection; (iv) item reduction; and (v) creation of a final version. Consensus about items that have to be part will be reached in a final consensus conference. Results. During a meeting in February 2009, we coordinated the development process of the health index for patients with AS. The results of this investigation will be the health index for patients with AS. Conclusion. The goal of developing a health index for patients with AS based on the ICF is very much in line with the broader goal of the WHO to define health indices to ensure the comparability of them within the framework of the ICF.