This paper describes one part of a major study aimed at building a substantive theory of the triage process. The paper reports on some early findings of a major component of the reasoning strategy utilized by nurses when making triage decisions via the telephone. The study reveals that in telephone triage nurses compensate for the absence of face-to-face contact by creating a mental image of the caller and the situation being dealt with. This forms the basis of the nurses' assessment of the urgency of the problem. The properties that contribute to this process of 'visualization work' are presented and the linkages with current thought in relation to clinical reasoning are explored.