Identification of patients with acute chest pain due to acute coronary syndrome is a common and difficult challenge for emergency physicians. A prospective study was conducted to assess the diagnostic value of a bedside test of cardiac troponin T in the emergency room setting. Forty-nine consecutive patients, who visited the emergency room within 6 hours of the onset of acute chest pain, were enrolled. Of the 26 patients who were ultimately diagnosed as having acute coronary syndrome, seven patients (27%) had positive cardiac troponin T assay results, whereas none of the patients without acute coronary syndrome had positive results (0%). For patients with acute coronary syndrome who presented later than 3 hours after the onset (n = 4), the test was positive in all cases (positive predictive value: 100%, negative predictive value: 100%, p < 0.01). However, the positive rate was only 14% for those who presented earlier than 3 hours after the onset (n = 22) (positive predictive value: 100%, negative predictive value: 47%, p = 0.84). In conclusion, bedside troponin T test results should be evaluated considering the time interval from the onset of chest symptoms.