Background: Every year an emergency medical technician or paramedic treats and transports up to several hundred patients. Only some patients are acutely seriously ill, and a few of these show only discrete signs and symptoms of their condition. This study aims to describe patients who died within 48 h of being admitted non-emergently to hospital by ambulance, examine the extent to which critically ill patients are recognized prehospitally, and identify clinical warning signs that might be frequently overlooked. Method: Registry based follow-up study on patients receiving an ambulance from the Copenhagen EMS in 2018. Data was included regarding the dispatch of the ambulance from the emergency services disposition system, ICD-10 hospital admission diagnoses from the National Patient Register, 48-h mortality from the Central Person Register and assessment and treatment in the ambulance by reviewing the electronic pre-hospital patient record. Results: In 2018 2279 patients died within 48 h after contact with the EMS, 435 cases met inclusion criteria. The patients’ median age was 83 years (IQR 75–90), and 374 (86.0%) had one or more underlying serious medical conditions. A triage category based on vitals and presentation was not assigned by the EMS in 286 (68.9%) cases, of which 38 (13.3%) would meet red and 126 (44.1%) orange criteria. For 409 (94.0%) patients, it was estimated that death within 48 h could not have been avoided prehospitally, and for 26 (6.0%) patients it was uncertain. We found 27 patients with acute aortic syndrome as admission diagnosis, of these nine (33.3%) had not been admitted urgently to a hospital with vascular surgery specialty. Conclusions: It was estimated that death within 48 h could generally not be avoided prehospitally. The patients’ median age was 83 years, and they often had serious comorbidity. Patients whose vital parameters met red or orange triage criteria were to a lesser degree triaged prehospitally, compared to patients in the yellow or green categories. Patients with acute aortic syndrome were not recognized by EMS 33.3% of the time.