This paper describes requirements engineering for a new pervasive health care system for hospitals in Denmark. The chosen requirements engineering approach composes iterative prototyping and explicit environment description in terms of work.ow modelling. New work processes and their proposed computer support are represented via a combination of prose, formal models, and animation. The representation enables various stakeholders to make interactive investigations of requirements for the system in the context of the envisioned work processes. The paper describes lessons learned from collaboration between users and system developers in engineering the requirements for the new system.