In 2014, a telemedicine system was established in 24-h routine use in the emergency medical service (EMS) of the city of Aachen. This study tested whether the diagnostic concordance of the tele-EMS physician reaches the same diagnostic concordance as the on-site-EMS physician. The initial prehospital diagnoses were compared to the final hospital diagnoses. Data were recorded retrospectively from the physicians' protocols as well as from the hospital administration system and compared. Also, all diagnostic misconcordance were analysed and reviewed in terms of logical content by two experts. There were no significant differences between the groups in terms of demographic data, such as age and gender, as well as regarding the hospital length of stay and mortality. There was no significant difference between the diagnostic concordance of the systems, except the diagnosis "epileptic seizure". Instead, in these cases, "stroke" was the most frequently chosen diagnosis. The diagnostic misconcordance "stroke" is not associated with any risks to patients' safety. Reasons for diagnostic misconcordance could be the short contact time to the patient during the teleconsultation, the lack of personal examination of the patient by the tele-EMS physician, and reversible symptoms that can mask the correct diagnosis.