Persons who have survived a stroke might lower the risk of having recurrent strokes by adopting a healthier lifestyle with more exercise. One way to promote exercising is by fitness or exergame apps for mobile phones. Health and fitness apps are used by a significant portion of the consumers, but these apps are not targeted to stroke survivors, who may experience cognitive limitations (like fatigue and neglect), have problems with mobility due to hemiplegia, and balance problems. We outline the design process, implementation and user involvement in the design of an exergame app that is specifically targeted to stroke survivors, and present the lessons learned during the design process.