Objective To examine the relation between time from onset of symptoms and coming under ambulance and hospital care on fatality in patients with evolving acute myocardial infarction, and on the proportions who survive because of resuscitation and thrombolytic treatment.

Design Prospective community and hospital study over two years. Delay was measured from the onset of symptoms to arrival at hospital, and from the onset to coming under care from ambulance personnel.

Setting Four general hospitals serving three United Kingdom health districts.

Patients 2213 patients under 75 years of age, 111 of whom had been successfully resuscitated from out of hospital cardiac arrest.

Interventions Resuscitation from cardiac arrest; thrombolytic treatment.

Main outcome measures 30 day fatality and lives saved by the two forms of treatment.

Results Times from symptom onset to coming under hospital care and to starting thrombolytic treatment (given to 53% of patients) were ⩽ 1 hour in 15% and 2% of patients respectively, ⩽ 2 hours in 54% and 25%, and ⩽ 4 hours in 67% and 55%. Overall, 30 day fatality was 138/1000 patients treated; 64/1000 (95% confidence interval 54 to 74) survived because of treatment, and 80% of this salvage was attributable to resuscitation. Delay was an important factor: 107/1000 (60 to 144) lives were saved for those coming under care within 1 hour compared with 21/1000 (5 to 37) for those who delayed for more than 12 hours. Further analysis including the 111 patients with out of hospital arrest showed that 34% of those coming to hospital by ambulance came under ambulance care within 1 hour; for this subset, 30 day fatality was 173/1000, and 136 (109 to 163) lives were saved by treatment.

Conclusions Results of treatment are strongly related to delay in coming under care. Reduction in delay can reduce mortality from acute myocardial infarction.