AIM: To evaluate factors which, prior to hospital admission, predict the development of acute coronary syndrome or acute myocardial infarction among patients who call for an ambulance due to suspected acute coronary syndrome. DESIGN: Prospective observational study. METHODS: All the patients who called for an ambulance due to suspected acute coronary syndrome in South Hospital's catchment area in Stockholm and in the Municipality of Göteborg between January and November 2000, were included. On arrival of the ambulance crew, a blood sample was drawn for bedside analysis of serum myoglobin, creatine kinase (CK)MB and troponin-I. A 12-lead electrocardiogram (ECG) was simultaneously recorded. RESULTS: In all, 538 patients took part in the survey. Their mean age was 69 years and 58% were men. In all, 307 patients (57.3%) had acute coronary syndrome and 158 (29.5%) had acute myocardial infarction. Independent predictors of the development of acute coronary syndrome were a history of myocardial infarction (P=0.006), angina pectoris (P=0.005) or hypertension (P=0.017), ECG changes with ST elevation (P<0.0001), ST depression (P<0.0001) or T-wave inversion (P=0.012) and the elevation of CKMB (P=0.005). Predictors of acute myocardial infarction were being a man (P=0.011), ECG changes with ST elevation (P<0.0001) or ST depression (P<0.0001), the elevation of CKMB (P<0.0001) and a short interval between the onset of symptoms and blood sampling (P=0.010). CONCLUSION: Among patients transported by ambulance due to suspected acute coronary syndrome, predictors of myocardial damage can be defined prior to hospital admission on the basis of previous history, sex, ECG changes, the elevation of biochemical markers and the interval from the onset of symptoms until the ambulance reaches the patient.