The incidence of contrast-induced nephropathy (CIN) in primary percutaneous coronary intervention (PCI) for ST-segment elevation myocardial infarction (STEMI) was previously reported to be as high as 19%. Iso-osmolar contrast has frequently been used for populations at high risk for CIN, but a recent meta-analysis did not show a significant benefit of using iso-osmolar contrast in preventing CIN. The aim of our study is to evaluate the impact of iso-osmolar contrast use in patients undergoing emergent PCI. We performed a retrospective analysis of patients who underwent primary and rescue PCI for STEMI. The PCI strategy, including the contrast choice, was left at the discretion of the operator. CIN was defined as an increase in creatinine of more than 0.5 mg or 25% from the baseline within 72 hours. Among 212 patients, CIN was seen in 33 patients (16%). Patients who received iso-osmolar contrast were older, and included more patients at risk for CIN. The incidence of CIN was 14% in the low-osmolar contrast group and 17% in the iso-osmolar contrast group (P=.799). After logistic regression analysis, CIN was seen more frequently in patients who had lower ejection fraction, post-PCI TIMI flow <3, and lower hemoglobin. The use of iso-osmolar contrast was not associated with a lower incidence of CIN in patients undergoing emergent PCI for STEMI.