UNLABELLED: The presence of thrombus is associated with adverse clinical outcomes. Our aim was to develop a classification of thrombus burden (TB) in patients with ST-segment elevation myocardial infarction (STEMI).METHODS: We retrospectively analyzed 900 consecutive patients treated with percutaneous coronary intervention for STEMI. Drug-eluting stents were used in 90.1%. TB was graded (G) as G0 = no thrombus, G1 = possible thrombus, G2 = small [greatest dimension ≤ 1/2 vessel diameter (VD)], G3 = moderate (> 1/2 but < 2VD), G4 = large (≥ 2VD), G5 = unable to assess TB due to vessel occlusion. Patients with G5 were reclassified to a thrombus category after flow achievement either with a guidewire or a small (1.5 mm) balloon. The incidence of major adverse cardiac events (MACE) - defined as death, myocardial infarction and infarct-related artery revascularization - was computed using the Kaplan-Meier method.RESULTS: Median duration of follow-up was 18.5 months. G5 patients constituted 57.7% of all patients and reclassification was achieved in 97.9%. TB after reclassification was G0, 8.1%; G1, 19%; G2, 24.5%, G3,16.6%, G4, 30%, G5, 1.9%. The 2-year cumulative MACE-free survival was comparable in G1, G2, G3 (84.5%, 85.9% and 87% respectively, p = 0.83), while G0 (75.8%) and G4 (75%) did significantly worse (p = 0.001). After stratification in two groups of small (G0-3) and large (G4) TB, the latter was found to be an independent predictor for 2-year mortality (HR: 1.66, 95% CI: 1.04-2.68, p = 0.035) and MACE rate (HR: 2.04, 95% CI: 1.44-2.88, p < 0.001).CONCLUSIONS: In patients with STEMI, TB can be reliably estimated in occluded infarct-related arteries. Large thrombus (≥ 2 VD) is a significant independent predictor for mortality and MACE.