BACKGROUND: Randomized trials comparing fibrinolysis with primary angioplasty for acute ST-elevation myocardial infarction have demonstrated a beneficial effect of primary angioplasty on the combined end point of death, reinfarction, and disabling stroke but not on all-cause death. Identifying a patient group with reduced mortality from an invasive strategy would be important for early triage. The Thrombolysis in Myocardial Infarction (TIMI) risk score is a simple validated integer score that makes it possible to identify high-risk patients on admission to hospital. We hypothesized that a high-risk group might have a reduced mortality with an invasive strategy. METHODS AND RESULTS: We classified 1527 patients from the Danish Multicenter Randomized Study on Fibrinolytic Therapy Versus Acute Coronary Angioplasty in Acute Myocardial Infarction (DANAMI-2) trial with information for all variables necessary for calculating the TIMI risk score as low risk (TIMI risk score, 0 to 4) or high risk (TIMI risk score > or =5) and investigated the effect of primary angioplasty versus fibrinolysis on mortality and morbidity in the 2 groups. Follow-up was 3 years. We classified 1134 patients as low risk and 393 as high risk. There was a significant interaction between risk status and effect of primary angioplasty (P=0.008). In the low-risk group, there was no difference in mortality (primary angioplasty, 8.0%; fibrinolysis, 5.6%; P=0.11); in the high-risk group, there was a significant reduction in mortality with primary angioplasty (25.3% versus 36.2%; P=0.02). CONCLUSIONS: Risk stratification at admission based on the TIMI risk score identifies a group of high-risk patients who have a significantly reduced mortality with an invasive strategy of primary angioplasty.