IMPORTANCE: About 10% of patients with acute chest pain are ultimately diagnosed with acute coronary syndrome (ACS). Early, accurate estimation of the probability of ACS in these patients using the clinical examination could prevent many hospital admissions among low-risk patients and ensure that high-risk patients are promptly treated. OBJECTIVE: To review systematically the accuracy of the initial history, physical examination, electrocardiogram, and risk scores incorporating these elements with the first cardiac-specific troponin. STUDY SELECTION: MEDLINE and EMBASE were searched (January 1, 1995-July 31, 2015), along with reference lists from retrieved articles, to identify prospective studies of diagnostic test accuracy among patients admitted to the emergency department with symptoms suggesting ACS. DATA EXTRACTION AND SYNTHESIS: We identified 2992 unique articles; 58 met inclusion criteria. MAIN OUTCOMES AND MEASURES: Sensitivity, specificity, and likelihood ratio (LR) of findings for the diagnosis of ACS. The reference standard for ACS was either a final hospital diagnosis of ACS or occurrence of a cardiovascular event within 6 weeks. RESULTS: The clinical findings and risk factors most suggestive of ACS were prior abnormal stress test (specificity, 96%; LR, 3.1 [95% CI, 2.0-4.7]), peripheral arterial disease (specificity, 97%; LR, 2.7 [95% CI, 1.5-4.8]), and pain radiation to both arms (specificity, 96%; LR, 2.6 [95% CI, 1.8-3.7]). The most useful electrocardiogram findings were ST-segment depression (specificity, 95%; LR, 5.3 [95% CI, 2.1-8.6]) and any evidence of ischemia (specificity, 91%; LR, 3.6 [95% CI,1.6-5.7]). Both the History, Electrocardiogram, Age, Risk Factors, Troponin (HEART) and Thrombolysis in Myocardial Infarction (TIMI) risk scores performed well in diagnosing ACS: LR, 13 (95% CI, 7.0-24) for the high-risk range of the HEART score (7-10) and LR, 6.8 (95% CI, 5.2-8.9) for the high-risk range of the TIMI score (5-7). The most useful for identifying patients less likely to have ACS were the low-risk range HEART score (0-3) (LR, 0.20 [95% CI, 0.13-0.30]), low-risk range TIMI score (0-1) (LR, 0.31 [95% CI, 0.23-0.43]), or low to intermediate risk designation by the Heart Foundation of Australia and Cardiac Society of Australia and New Zealand risk algorithm (LR, 0.24 [95% CI, 0.19-0.31]). CONCLUSIONS AND RELEVANCE: Among patients with suspected ACS presenting to emergency departments, the initial history, physical examination, and electrocardiogram alone did not confirm or exclude the diagnosis of ACS. Instead, the HEART or TIMI risk scores, which incorporate the first cardiac troponin, provided more diagnostic information.