Among 4,333 patients who were triaged in the emergency department (ED) over a 1-year period in 2003 because of acute chest pain, 1,747 (40%) were stratified as "low risk" on the basis of a Thrombolysis In Myocardial Infarction (TIMI) risk score of 0 to 2. Results showed that, during ED stay, TIMI risk score increased to > or =3 in 63% of patients and that such patients were more likely to be diabetic, hypertensive, hyperlipidemic, and smokers, and to have had previous myocardial infarction or revascularization. Patients with changes in TIMI risk score were admitted more often to the hospital, whereas more patients with unchanged TIMI risk score were discharged home directly from the ED. In conclusion, TIMI risk score may change soon after arrival to the ED in 50% of patients with acute chest pain who are initially triaged as low risk. Changes in TIMI risk score are more common in patients with multiple risk factors and/or previous diagnosis of coronary artery disease. Serial, frequent assessments of TIMI score during the ED observation period are mandatory, particularly in these subsets of patients.