A history of diabetes mellitus (DM) is an independent predictor for adverse events in patients with acute ST-segment elevation myocardial infarction (STEMI) undergoing primary percutaneous coronary intervention. Outcomes of patients with STEMI and newly diagnosed DM (NDM) are less well described. We used the Harmonizing Outcomes with Revascularization and Stents in Acute Myocardial Infarction (HORIZONS-AMI) trial database to identify the outcomes at 30 days and 3 years according to no, known, and NDM in patients with STEMI. In HORIZONS-AMI, 3,602 patients with STEMI were randomized to bivalirudin versus heparin and a glycoprotein IIb/IIIa inhibitor, and eligible patients were randomized again to a paclitaxel-eluting stent or a bare-metal stent. DM was defined as a history of hyperglycemia managed by insulin, oral hypoglycemic agents, or diet. NDM was defined as the absence of previous diagnosis or treatment for DM at baseline and its addition at discharge. DM was present in 593/3,599 patients (16.5%), and NDM was diagnosed in 130 cases (3.6%). Compared with nondiabetics, those with DM and NDM had higher 3-year rates of death (11.4% and 12.0% vs 5.6%, respectively, p <0.0001) and major adverse cardiac events (29.6% and 30.2% vs 19.9%, respectively, p <0.0001). There were no significant differences in adverse events between new and known diabetic patients. DM and NDM were independent predictors of 3-year mortality and 3-year major adverse cardiac events. In conclusion, patients with NDM have a similarly poor prognosis after primary percutaneous coronary intervention in STEMI as those with previously established DM.