Renal dysfunction and its change pattern are associated with short- and long-term mortality. However, it remains to be investigated whether or not worsening renal function (WRF) defined by baseline renal function identified from different time points would provide prognostic implication on outcomes in acute coronary syndrome (ACS) patients. This study consists of 334 ACS patients (mean age 68 ± 11 years, 75% male) treated with emergent percutaneous coronary intervention (PCI). Estimated glomerular filtration rate (eGFR) was evaluated on baseline, during hospitalization, at discharge, and at 3-month follow-up. WRF was defined as a relative decrease of eGFR > 20% at 3 months using baseline eGFR identified from different time points. The primary end point was a composite event of major cardiovascular events (MACE), including all-cause death, ACS, and heart failure hospitalization. The associations of chronic kidney disease (CKD), acute kidney injury (AKI), and WRF with MACE were evaluated. During a mean follow-up of 3.3 ± 1.7 years, a total of 64 MACE were observed. Multivariable analysis revealed that CKD (hazard ratio 2.16; p = 0.018) and AKI (hazard ratio 1.95; p = 0.030) were independent predictors of MACE, but WRF did not remain as an independent predictor of MACE (p = 0.208). The highest risk was observed in AKI patients with CKD when stratified by the presence or absence of CKD and AKI. In ACS patients treated with emergent PCI, this study demonstrated that CKD and AKI were independent predictors of MACE, while there was no independent relationship between WRF and MACE.