Many software experts argue that when we design a new system, we should create an explicit description of the environment in which the proposed system is to be used. The argument becomes crucial for pervasive computing, which aims to tightly integrate systems into their environments and into the work processes they're to support. However, prototypes typically provide an explicit representation only of the system itself. Executable use cases, on the other hand, can also describe the environment. EUCs are designed to: narrow the gap between informal ideas about requirements and the formalization that eventually and inevitably emerges during system implementation; and spur communication between users and system developers. Iterative prototyping is a feasible starting point to pursue these goals. But EUCs encompass both iterative prototyping and explicit environment descriptions in terms of workflow modeling. This article describes a case study in which developers used EUCs to prototype an electronic patient record system for hospitals in Aarhus, Denmark.