The purpose of this study was to elicit the components of diagnostic reasoning utilized by experienced triage nurses when making triage dispositions via the telephone. Five first-level nurses triaged two simulated calls portrayed by the author. The resultant interactions were recorded on an answer-phone and subsequently replayed to the relevant participant. The tape was stopped after each question or comment made by the nurse who was then asked to say what she was thinking at the time. These verbalizations were audio-taped, transcribed and subjected to analysis using grounded theory. Although deprived of the opportunity for methodical deliberation, nurses, when making decisions, did so within a systematic and identifiable framework. The process was found to centre around five major considerations; the most likely cause of the presenting problem, the impact of the problem upon the caller, the accessibility of alternative sources of health care, the ability of the nurse to control the reactions of the caller and the nurses' perception of their vulnerability. The final triage disposition was made through balancing the most probable outcome against the worst possible outcome.