OBJECTIVES: To evaluate the setting of priorities and patients' need for the ambulance service. METHODS: A prospective, consecutive study was conducted during a 6-week period. The ambulance staff completed a questionnaire assessing each patient's need for prehospital care. In addition to the questionnaire, data were extracted from the ambulance medical records for each case. RESULTS: The study included 1977 ambulance assignments. The results show that there is a substantial safety margin in the priority assessments made by the emergency medical dispatch operators, where the ambulance staff support the safety margin for initial priorities, despite the lack of at-the-scene confirmation. At-the-scene assessments indicated that 10% of all patients had potentially life-threatening conditions or no signs of life, but the advanced life support units were not systematically involved in these serious cases. The results even showed that one-third of the patients for whom an ambulance was assigned did not need the ambulance service according to the assessment made by the ambulance staff. CONCLUSION: Using the criteria-based dispatch protocol, the personnel at the emergency medical dispatch centres work with a safety margin in their priority assessments for ambulance response. Generally, this 'overtriage' and safety margin for initial priority settings were supported as appropriate by the ambulance staff. According to the judgement of the ambulance staff, one-third of all the patients who were assigned an ambulance response did not require ambulance transport.