In this paper we study the application of simulation as a method to support information system and process design in complex organizational transitions. We apply a combined use of a collaborative workshop approach with the use of a detailed and accurate graphical simulation model in a hospital that is in a major transition. The simulation represents the future situation of the hospital and enables the healthcare professionals to analyze and reflect on processes, planning, staffing and collaboration, and the new organizational processes and systems. We follow an action research setup in which, conducting iterative workshops, our simulation model is enhanced and fine-tuned. The data collected from the workshops reveal that the benefits of using simulations in complex organizational transitions include a substantial increase of awareness, insights and acceptance by the medical and administrative staff.