Introduction: The prevalence of diabetes mellitus (DM) in patients with myocardial infarction (MI) is estimated at about 25%. Short- and long-term prognosis is worse in patients with DM and hyperglycemia compared with nondiabetics. Objectives: Our aim was to analyze the impact of DM and transient hyperglycemia on in-hospital complications and long-term outcomes in patients with MI. Patients and methods: We evaluated a prospective cohort of 58 394 patients with MI in the Polish Registry of Acute Coronary Syndromes (PL-ACS) in terms of in-hospital complications and 30-day, 12-month, and 36-month outcomes between 2009 and 2012. Results: Patients with type 1 DM (T1DM) and type 2 DM (T2DM) underwent invasive diagnostic and therapeutic procedures less frequently than those without diabetes, transient hyperglycemia, and new-onset DM (P <⁠0.001). T2DM was associated with a significantly higher risk of MI complicated by cardiogenic shock. The rates of pulmonary edema in the transient hyperglycemia group and the DM group were 2-fold higher than in the nondiabetic group (P <⁠0.001). T1DM and T2DM were associated with a significantly higher risk of death. Unadjusted 3-year all-cause death rates in patients with T1DM, T2DM, transient hyperglycemia, new-onset diabetes and those without diabetes were 26.8%, 25.6%, 18.5%, 17.9%, and 16.2%, respectively. Hazard ratios (95% CI) adjusted for age, sex, clinical characteristics, and revascularization were 1.49 (1.12­2.00), 1.20 (1.14­1.27), 0.94 (0.67­1.31), and 0.66 (0.34­1.28), respectively. Conclusions: T1DM and T2DM are associated with elevated in-hospital and long-term mortality rates after MI. Diabetics and patients with transient hyperglycemia are more likely to develop significant in-hospital complications compared with nondiabetics. No significant differences regarding acute mechanical complications were noted between populations.