Many people visit an emergency department immediately after experiencing some unusual symptoms, without any regard to the scale of acuity of their condition. Some of these visits are unnecessary and lead to overcrowded emergency departments, increased wait times, and overutilization of scarce healthcare resources. At the other end of the spectrum, there are people who do not seek medical advice when they should. One solution to these problems is to provide a reliable source of information that would allow people to assess their need for consulting (self-triage) in a way that helps reduce the number of inappropriate visits and that promotes seeking medical help when necessary. This paper aims to identify requirements that make such self-triage applications usable. To this end, after assessing existing apps, we studied a new self-triage mobile application called Symptoms Pal, co-developed with Montfort Hospital in Ottawa, Canada. This application guides a user through a series of questions and, at the end, provides advice about the possible problem and course of action (911, visit an emergency department, visit a doctor, or self-care). The usability of Symptoms Pal was evaluated by a study involving 34 participants. Several strengths and weaknesses of the usability and perceived usefulness of the application were identified and led to additional requirements and a revised version. This research contributes 25 reusable requirements and validated user interface design artefacts for self-triage mobile applications that can be used by developers of similar applications.