Abstract Funding Acknowledgements Type of funding sources: None. Background Acute kidney injury (AKI) in acute coronary syndrome (ACS) patients is a well-known marker of worse prognosis. However, it remains unclear how timing of AKI development correlates with mortality and morbidity; Objective Assess the timing of AKI development and evaluate its short and long term prognostic impact. Methods Retrospective study of patients with ACS periodically included in our center registry between March/2013 and December/2018. AKIwas defined as increase in creatinine ≥0.3 mg/dl or ≥50% within 48 h after admission. Patients were classified into 3 groups according to the occurrence and timing of AKI development : no-AKI(NA), early-AKI(EA)(<48h) and late-AKI(LA) (>48 h). The primary endpoints were all cause mortality and a composite of all cause mortality, nonfatal myocardial infarction/stroke and readmission in the follow-up. Results We included 518 patients (67 ± 13 years; 73% males, 46% STEMI) of whom 17% developed AKI(8% EA and 9% LA). Patients with AKI, particularly EA, were older (NA: 67± 17; EA 80± 12; LA 74 ± 16, p< 0.001), had more hypertension, previous heart failure (9% vs 1.5%, p < 0.001) and dementia (7 vs 0.5%, p < 0.001). Coronariograhy was performed more often in NA patients (98%), followed by LA (100%) and lastly EA (87.5%), p < 0.001. During hospitalization, those who developed AKI had a higher Killip Kimball class (p = 0.043) and lower ejection fraction (EF) (p = 0.05). In-hospital mortality was significantly higher in patients with EA (12,5%) than LA (2.2%) or LA (2.3%)( p= 0.029) ; During a median follow-up of 35 months, the composite endpoint was particularly higher in the EA group, but no differences were found between LA and NA group. In multivariate analysis, only EA was an independent predictor of all-cause mortality (HR: 3.8 IC 95% 1.8-8.1, p = 0.001) and composite endpoint (HR:2.02 IC95%1.1-3.8; p = 0.032), even after adjusting for age, EF and Killip Kimball class; Conclusion In this population of ACS patients, AKI is a frequent complication and the timing of its development has major prognostic implications, since early AKI(<48h) is associated with worse outcomes. Curiously, mortality and CV events in patients with LA are not different from those who do not develop AKI.