This study was performed to evaluate the effects of diabetes on short- and mid-term clinical outcomes in patients with acute myocardial infarction (AMI). Between October 2005 and December 2009, a total of 22,347 patients with AMI from a nationwide registry was analyzed. At the time point of the day 30 after AMI onset, landmark analyses were performed for the development of major adverse cardiovascular events (MACEs), including death, re-infarction and revascularization. In this cohort, 6,131 patients (27.4%) had diabetes. Short-term MACEs, which occurred within 30 days of AMI onset, were observed in 1,364 patients (6.1%). Among the 30-day survivors (n = 21,604), mid-term MACEs, which occurred between 31 and 365 days after AMI onset, were observed in 1,181 patients (5.4%). After adjustment for potential confounders, diabetes was an independent predictor of mid-term MACEs (HR, 1.25; 95% CI, 1.08-1.45; P = 0.002), but not of short-term MACEs (HR: 1.16; 95% CI: 0.93-1.44; P = 0.167). Diabetes is a poor prognostic factor for mid-term clinical outcomes but not for short-term outcomes in AMI patients. Careful monitoring and intensive care should be considered in diabetic patients, especially following the acute stage of AMI.