OBJECTIVE To determine the effects of oxygen therapy in myocardial infarction (MI) patients with and without diabetes. RESEARCH DESIGN AND METHODS In the Determination of the Role of Oxygen in Suspected Acute Myocardial Infarction (DETO2X-AMI) trial, 6,629 normoxemic patients with suspected MI were randomized to oxygen at 6 L/min for 6-12 h or ambient air. In this prespecified analysis involving 5,010 patients with confirmed MI, 934 had known diabetes. Oxidative stress may be of particular importance in diabetes, and the primary objective was to study the effect of supplemental oxygen on the composite of all-cause death and rehospitalization with MI or heart failure (HF) at 1 year in patients with and without diabetes. RESULTS As expected, event rates were significantly higher in patients with diabetes compared with patients without diabetes (main composite end point: hazard ratio [HR] 1.60 [95% CI 1.32-1.93], P amp;lt; 0.01). In patients with diabetes, the main composite end point occurred in 16.2% (72 of 445) allocated to oxygen as compared with 16.6% (81 of 489) allocated to ambient air (HR 0.93 [95% CI 0.67-1.27], P = 0.81). There was no statistically significant difference for the individual components of the composite end point or the rate of cardiovascular death up to 1 year. Likewise, corresponding end points in patients without diabetes were similar between the treatment groups. CONCLUSIONS Despite markedly higher event rates in patients with MI and diabetes, oxygen therapy did not significantly affect 1-year all-cause death, cardiovascular death, or rehospitalization with MI or HF, irrespective of underlying diabetes, in line with the results of the entire study.