© 2017 The Authors The utility of the electrocardiogram (ECG) in patients with acute left circumflex (LC) coronary occlusion is not established. This study aimed at determining the clinical, angiographic, and prognostic characteristics associated with the different patterns of ST-segment changes in patients with LC occlusion. A cohort of 314 patients with LC occlusion was categorized according to the admission ECG: (1) ST-segment elevation (ST-E, n=208), (2) isolated ST-segment depression in precordial leads (ST-D, n=62), and (3) negligible ST-segment changes (No-ST, n=44). Clinical variables, coronary angiography, and 30-day major adverse cardiac event (MACE) (in-hospital ventricular fibrillation, 1-month mortality, or heart failure) were compared among the three groups. As compared with No-ST, patients with ST-E or ST-D presented more advanced Killip class, higher troponin peak, lower LV ejection fraction, and were independently associated with MACE (odds ratio 5.43, 95% confidence interval 1.09 to 27.20 and odds ratio 3.39, 95% confidence interval 0.66 to 17.50, respectively). Patients with ST-D were tardily reperfused, had more often mitral regurgitation (23.1% vs 9.3% in ST-E and 3.3% in No-ST, p=0.03), and presented ST-segment elevation in leads V7 to V9 in 12 of 16 cases with available recordings. Culprit proximal LC predominated in ST-D (41.9%), distal LC in ST-E (42.8%), and obtuse marginal in No-ST (59.1%) (all p<0.01). The No-ST had smaller coronary vessels and more collaterals. In conclusion, the three ST-segment patterns of LC occlusion identify patients with different clinical, angiographic, and prognostic characteristics. Patients with ST-depression pattern require a prompt reperfusion therapy and could be better recognized by recording leads V7 to V9.