AIM: To evaluate factors that identify patients with an acute coronary syndrome/myocardial infarction prior to hospital admission among patients with a suspected acute coronary syndrome who were transported by ambulance with and without ST elevation on the ambulance electrocardiogram (ECG). METHODS: This was a prospective observational study in the part of Stockholm that is served by South Hospital ambulance organisation and the Municipality of Goteborg. All the patients who called for an ambulance due to acute chest pain or other symptoms raising the suspicion of an acute coronary syndrome took part. Immediately after the arrival of the ambulance, a blood sample was drawn for the analysis of serum myoglobin, creatine kinase (CK) MB and troponin I. A 12-lead ECG was simultaneously recorded. Further factors that were taken into consideration were age, gender, history of cardiovascular disease, symptoms and clinical findings. RESULTS: In patients with ST elevation in prehospital ECG, the likelihood of an acute myocardial infarction increased if there were simultaneous ST depression in other leads (OR 3.94, 95% CL 1.26-12.38). For patients without an ST elevation, the likelihood of an acute myocardial infarction increased if there were: elevation of any biochemical marker OR 2.96, 95% CL 1.32-6.64; ST depression (OR 2.54, 95% CL 1.43-4.51), T-inversion (OR 2.22, 95% CL 1.10-4.48), male gender (OR 2.21, 95% CL 1.24-3.93) and increasing age (OR 1.04, 95% CL 1.01-1.06). CONCLUSION: Among patients with a suspected acute coronary syndrome, factors that increased the likelihood for an ongoing acute myocardial infarction could already be defined prior to hospital admission. For those with an ST elevation, factors were found in ECG pattern. For those without an ST elevation, such factors were found in elevation of biochemical markers, admission ECG, male gender and increasing age.