Limited data exist on the role of coronary collaterals circulation (CCC) in patients with ST-elevation acute coronary syndrome (STE-ACS). This study aimed to assess CCC and the in-hospital course of patients with CCC undergoing primary percutaneous coronary intervention (pPCI). The study included consecutive STE-ACS patients undergoing pPCI. Good CCC was defined as Rentrop collateral score (RCS) of 2-3. Patients with good and poor CCC were compared regarding clinical characteristics, angiographic patterns, and hospital course. In the sample of 4,683 patients, mean age was 55.6±11 years, and 78.8% were male. Good CCC was observed in 499 (10.7%) patients. The rate of intra-procedure slow-flow/no-reflow (SF/NR) was 29.9% vs. 20.5% (p<0.001), and the rate of composite adverse clinical outcomes (CACO) was 21.2% vs. 19% (p=0.225) for patients with good and poor CCC, respectively. Multivariable analysis identified left ventricular end-diastolic pressure (LVEDP), multi-vessel disease (MVD), and thrombus grade ≥4 as independent predictors of good CCC, with adjusted odds ratios of 0.98 [0.97-0.99], 1.69 [1.35-2.10], and 3.45 [2.64-4.52], respectively. In propensity-matched cohorts, the intra-procedure SF/NR rate was 29.9% vs. 26.9% (p=0.292), and the rate of CACO was 21.2% vs. 23.4% (p=0.403) for patients with good and poor CCC, respectively. Angiographic evidence of good CCC in STE-ACS patients was limited. Good CCC was associated with a higher prevalence of MVD, high thrombus burden, and low pre-procedure LVEDP, resulting in a higher incidence of intra-procedure SF/NR. However, the CACO did not differ significantly between patients with good and poor CCC.