For most medical informatics software products, insufficient effort is spent on the design phase of production. However, poor design often leads to systems that are either not well accepted, or far less effective than they could be. In this paper, we describe the ideas of participatory design and discuss why these ideas are especially applicable to medical informatics systems. In particular, we present a case study in the area of clinical trial protocol management. We designed and developed a tool aimed at increasing accrual to clinical trial protocols at an oncology center. However, the design evolved over time, and features of this design were only discovered through iterative development and interaction with the users within the context of the workplace.