Objective: The aim of this study was to assess the myocardial energy expenditure (MEE) in patients with cardiac syndrome X (CSX) and to examine its association with exercise electrocardiogram (ECG) parameters.,Methods: A total of 99 patients who underwent coronary angiography and who were diagnosed as having normal coronary arteries were included. The patients were divided into 2 groups based on symptoms and exercise ECG parameters: 56 CSX patients and 43 control patients with a negative stress test. MEE was calculated using transthoracic echocardiography-derived parameters: circumferential end-systolic stress, left ventricular ejection time, and stroke volume.,Results: In patients with CSX, the MEE at rest was 28% higher in than the control group (89.2 +/- 36.3 vs. 69.8 +/- 17.2 cal/minute). Correlation analysis revealed a moderately negative correlation between MEE and the Duke treadmill score (DTS) beta:-0.456; p<0.001). Receiver operating characteristic analysis with a cut-off value of 74.6 cal/minute for MEE had a sensitivity of 78.1% and a specificity of 75.3% for the prediction of CSX (area under the curve: 0.872; p<0.001). An extra 1 calorie spent per minute at rest increased the likelihood of CSX by about 86% (odds ratio: 1.863).,Conclusion: This study demonstrated that MEE was greater in CSX patients compared with a control group. Increased MEE was determined to be an independent predictor of CSX. DTS was inversely correlated with MEE. Increased MEE may have a crucial role in CSX pathophysiology.