This study explores a method for the co-design of mobile applications in the context of use. In 36 sessions with future users, synchronous co-design of a mobile navigation application was conducted in the intended use environment – a hospital – using an interactive Wizard-of-Oz-controlled prototype. The results show that co-design in the intended use environment contributes to the elicitation of design suggestions. Concerning the co-design method, the results show that by using interactive prototyping the user is actively involved as a co-designer, which empowers the user and enables the continuous evaluation of design suggestions.