OBJECTIVES: To evaluate the occurrence of elevation of serum biochemical markers for myocardial damage in the prehospital setting amongst patients who called for an ambulance due to a suspected acute coronary syndrome (ACS). DESIGN: Prospective observational study. SUBJECTS: All the patients who called for an ambulance due to suspected ACS. SETTING: South Hospital's catchment area in Stockholm and in the Municipality of Göteborg, Sweden between January and November in the year 2000, were included. INTERVENTIONS: On arrival of the ambulance crew, a blood sample was drawn for bedside analysis of serum myoglobin, creatine kinase MB and troponin I. A 12-lead electrocardiogram (ECG) was simultaneously recorded. MAIN OUTCOME MEASURES: Elevation of biochemical markers prior to hospital admission. RESULTS: In all, 511 patients participated on 538 occasions. Elevation of any biochemical marker was observed in 11% of all patients. The corresponding figure for patients developing myocardial infarction was 21%; for patients with myocardial ischaemia 8%; for patients with a possible myocardial ischaemia 4% and for patients with other diagnoses 5%. Amongst those who had a final diagnosis of acute myocardial infarction (AMI), 47% had ST-elevation on initial ECG and 57% had either ST-elevation or elevation of any biochemical marker. CONCLUSION: Bedside analysis of biochemical markers in serum is already feasible prior to hospital admission amongst patients with a suspected ACS. About 20% of patients with AMI have elevated biochemical markers at that stage. When found this data might increase the possibility of diagnosing an AMI very early in the course. However, false positives were found and whether this strategy will improve the triage of these patients in the prehospital setting remains to be proven.