Brain–body interfaces (BBIs) have been shown through a number of studies to be useful assistive technology devices for recreation and communication. However, severely motor impaired persons with no other means of interacting with their environment have had difficulties using the standard communication software for the Cyberlink, the commercially available BBI device which we used in our studies. We have therefore developed a simple Yes/No program, drawing on a range of design and evaluation approaches from Human–Computer Interaction research. This paper presents the first extensive evaluation of this program. Its evaluation combines formal assessments with observations from users, carers and technology and mental health professionals who are involved in the research. Our conclusions are that the performance of severely impaired individuals cannot be readily separated from that of novice unimpaired users, that worthwhile results can be achieved with the Yes/No program, but at the moment the cost of using a Cyberlink is too high for most assistive technology contexts. However, for severely impaired individuals, Cyberlink use may be the only form of recreation and communication available to them, and thus the current limitations of the technology are acceptable for this user population.