Severe illnesses are not always diagnosed during telephone triage and this sometimes results in a fatal outcome. The legal judgement of these cases includes the assessment of the triage organisation and procedure. In addition, there is a medical assessment: should the triage nurse have estimated the urgency correctly? Retrospective research has shown that telephone triage cannot effectively identify patients with a severe illness, such as acute coronary syndrome or bacterial meningitis. Research into the origin and the prevention of diagnostic failures is necessary to provide a framework for the medico-legal judgment of putative failures.