Background Diabetes is associated with more severe prognosis in patient with AMI. This study was conducted to investigate the differential effects of diabetes on the clinical outcomes in patients with ST-elevation myocardial infarction (STEMI) compared to those with non-ST-elevation myocardial infarction (NSTEMI). Methods Between October 2005 to December 2009, a total of 16220 patients with acute myocardial infarction (AMI) were identified from the Korean AMI registry (KAMIR) and Korea myocardial infarction registry (KORMI). Of these patients, 15118 who had available information were analyzed in this study. Thirty-day mortality and 6-month and 12-month major adverse cardiovascular events (MACEs) including death, recurrent myocardial infarction and revascularization were recorded. Results Among patients in this analysis, 4214 (27.8%) had diabetes and 8803 (58.2%) presented with STEMI. Lower percentage of patients with STEMI had diabetes than patients with NSTEMI (25.2 vs. 31.5%, p <0.001). A total of 182 (1.2%) patients were dead until 30-day after AMI, and mortality was significantly higher among patients with diabetes than among without diabetes (1.8 vs. 1.0%, p <0.001) at 30-day following either STEMI (1.5 vs. 1.0%, p=0.043) or NSTEMI (2.1 vs. 0.9%, p <0.001). When multiple logistic regression analyses were performed after controlling confounders the significant impact of diabetes on 30-day mortality was disappeared in the patients with both STEMI and NSTEMI (p >0.05). Among 30-day survivors, a total of 434 patients (2.9%) had MACEs at 6-month follow-up. The adjusted risk of MACEs at 6-month associated with diabetes among patients presenting with STEMI was 1.52 (95% CI 1.13–2.04, p=0.005). However, diabetes was not a predictor of MACEs at 6-month among patients presenting with NSTEMI in this analysis (p=0.972). Among 30-day survivors, a total of 207 patients (1.4%) had MACEs at 12-month follow-up. MACEs at 12-month occurred more frequently among patients with diabetes presenting with STEMI (5.9 vs. 4.4%, p <0.001) but not among patients with NSTEMI (5.9 vs. 4.9%, p=0.569). At 12 month, diabetes remained an independent predictor of MACEs for patients presenting with STEMI (OR 1.55, 95% CI 1.18–2.04, p=0.002) but not among patients with NSTEMI (p=0.786) in multiple logistic regression models. Conclusions Diabetes is a poor prognostic factor predicting 6-month and 12-month MACEs in the patients with STEMI but not with NSTEMI. More careful monitoring should be focused on diabetic patients after STEMI.