Computerized symptom checkers are becoming increasingly popular, however, although they have been tested for accuracy there is little information around the usability and perceived usefulness of computer-based symptom checkers. Previous work has shown that elderly users prefer to use larger format devices. This study concentrated on the differences between smartphone and PC-based systems. This research compared four symptom checkers (two PC based: Isabel, Symcat and two smartphones based: Ada, WebMD) using a think-aloud usability evaluation protocol and a questionnaire based on the technology acceptance model (TAM).