Requirements capture by scenarios and user interface prototyping have become popular techniques. Yet, the transition from scenarios to formal specifications remains ill-defined, and prototyping remains weak in linking the application domain with the user interface. Most importantly, the prototyping and the scenario approaches lack integration in the overall requirements engineering process. In this paper, we suggest a process that generates a user interface prototype from scenarios and yields a formal specification of the application. The approach is based on the Unified Modeling Language (UML), and the generated prototypes are embedded in a user interface builder environment for further refinement. The algorithms underlying the approach have been implemented and applied on a number of examples.