This paper describes the collaborative process of building a simulation model in order to understand patient waiting times in an accident and emergency department. The purpose is to explore the issues that arise when involving clients, in this case health care professionals, in the process of model building. The study background and a detailed account of the modelling process are presented. The focus of the paper is on some general themes that can be discovered running through the process. These offer some tentative insights into client involvement in system dynamics modelling, in particular its application in health care. Given this study's first promising results, further collaborative studies are encouraged.