Symptoms play a crucial part in the formulation of medical diagnoses, yet the construction and interpretation of symptom narratives is not well understood. The diagnosis of angina is largely based on symptoms, but a substantial minority of patients diagnosed with "non-cardiac" chest pain go on to have a heart attack. In this ethnographic study our aims were to understand: (1) how the patients' accounts are performed or enacted in consultations with doctors; (2) the ways in which ambiguity in the symptom narrative is managed by doctors; and (3) how doctors reach or do not reach a diagnostic decision. We observed 59 consultations of patients in a UK teaching hospital with new onset chest pain who had been referred for a specialist opinion in ambulatory care. We found that patients rarely gave a history that, without further interrogation, satisfied the doctors, who actively restructured the complex narrative until it fitted a diagnostic canon, detaching it from the patient's interpretation and explanation. A minority of doctors asked about chest pain symptoms outside the canon. Re-structuring into the canonical classification was sometimes resisted by patients who contested key concepts, like exertion. Symptom narratives were sometimes unstable, with central features changing on interrogation and re-telling. When translation was required for South Asian patients, doctors considered the history less relevant to the diagnosis. Diagnosis and effective treatment could be enhanced by research on the diagnostic and prognostic value of the terms patients use to describe their symptoms.