The relation between diabetes mellitus (DM) and outcome was assessed in a series of 1,061 patients with acute myocardial infarction (AMI) who underwent primary percutaneous coronary intervention (PCI). The efficacy of reperfusion was assessed by ST-segment resolution analysis. Of 1,061 patients, 166 had DM (15.6%), and 84 had insulin-requiring DM (51% of DM patients). The 6-month mortality rate was 26% in insulin-requiring DM patients, 7% in non-DM patients, and 4% in non-insulin-requiring DM patients (p <0.001). The early ST-segment resolution rate was lower in insulin-requiring DM patients (52%) compared with the other DM patients (78%) and non-DM patients (76%; p <0.001). Multivariate analysis showed insulin-requiring DM to be independently related to the risk for death (hazard ratio 1.94, 95% confidence interval 1.17 to 3.22, p = 0.009). Insulin-requiring DM is a strong predictor of mortality in patients who undergo PCI for AMI, and this relation may be explained by a less effective myocardial reperfusion despite the mechanical restoration of normal epicardial flow in most patients.