Patients with diabetes mellitus (DM) have an adverse prognosis after ST-segment elevation myocardial infarction (STEMI). Whether DM was associated with impaired myocardial reperfusion after successful primary percutaneous coronary intervention for STEMI was investigated. Myocardial reperfusion was assessed by ST-segment resolution and myocardial blush grade (MBG). A total of 386 patients were studied, of whom 64 (17%) had DM. These patients more frequently had reduced MBG (20% vs 10%, p = 0.02) and incomplete ST-segment resolution (55% vs 35%, p = 0.02) compared with patients without DM. After multivariate analysis, DM was still associated with impaired ST resolution (odds ratio 2.1, p = 0.03) and reduced MBG (odds ratio 2.2, p = 0.03).