Objective: European ST-segment elevation myocardial infarction (STEMI) guidelines recommend prehospital diagnosis to facilitate early reperfusion in patients with STEMI, and they provide recommendations regarding optimal system delay (time from first medical contact (FMC) to the primary percutaneous coronary intervention (PPCI)). There are limited data on achievable system delays in an optimal STEMI system of care using prehospital diagnosis to triage patients with STEMI directly to percutaneous coronary intervention (PCI) centres. We examined the proportion of tentative prehospital STEMI diagnoses established by telemedicine confirmed on hospital arrival, and we determined system delay in patients diagnosed before hospital arrival and triaged directly to the catheterisation laboratory. Methods: Design: Population-based follow-up study. Setting: Central Denmark Region. Participants: 15 992 patients diagnosed using telemedicine. Results: During the study period, a tentative diagnosis of STEMI was established in 1061 patients, of whom 919 were triaged directly to the PCI centre. In 771 (84%) patients, a diagnosis of STEMI was confirmed. Patients transported <10 km had a mean system delay of 82 min; this delay rose to 103 min for patients transported 50–75 km. Data on system delay was achievable in 682 patients in whom 553 (81%) were treated within 120 min of the FMC, and a system delay of <120 min was achievable in 89% of patients living up to 95 km from the PCI centre. Even for patients living <10 km from the PCI centre, only 16 (14%) had a system delay of 60 min or less. Conclusions: The use of telemedicine for prehospital diagnosis and triage of patients directly to the catheter laboratory is feasible and allows 89% of patients living up to 95 km from the invasive centre to be treated with PPCI within 120 min of the emergency medical service call. The study confirms that a recommendation of a system delay <60 min is unachievable if the FMC is the emergency medical call.