Background: Infarct size (IS) increases with vascular occlusion time, area at risk for infarction (AR), the lack of collateral supply (collateral flow index, CFI), absence of preconditioning, and with myocardial demand for oxygen supply. ECG ST segment elevation (STE) is used as a measure of severity of ischemia, and a surrogate for IS. This study in 50 patients with coronary artery disease undergoing a first 120-s balloon occlusion of a stenosis sought to determine whether STE corrected for the mentioned variables is different in the left (group LCA, n=36) and the right coronary artery (group RCA, n=14) territory. Methods and Results: Intracoronary ECG STE was obtained via the angioplasty pressure sensor guidewire. Absolute values of intracoronary ECG STE (STE, mV) as well as those relative to the respective R-amplitude (rSTE, i.e., regional mass correction) were obtained. AR was measured angiographically as percent of summed arterial branch lengths distal to the stenosis location relative to the entire LCA or RCA tree length (i.e., measure of stenosis proximity to the ostium). CFI (no unit) was determined by simultaneous mean aortic, coronary occlusive and central venous pressure measurements. STE at 120s was 0.27±0.13mV in the LCA group and 0.09±0.07mV in the RCA group (p<0.0001). rSTE at 120s was 0.27±0.17 in the LCA and 0.18±0.15 in the RCA group (p=0.04). AR and CFI were not statistically different between the groups. ECG RR interval at 120s of occlusion was 761±190ms respectively 922±177 in the LCA and RCA group (p=0.009). Conclusion: After accounting for all known determinants of infarct size, particularly mass at risk and collateral supply, the LCA territory is more sensitive to a 2-minute period of myocardial ischemia than the RCA region. This may be explained by a higher oxygen demand during occlusion in the LCA than the RCA territory.