The objective of the present study was to compare left ventricular (LV) function and clinical outcomes in diabetics versus nondiabetics with acute myocardial infarction (AMI) treated by primary coronary angioplasty. A total of 327 consecutive AMI subjects were reperfused by primary coronary angioplasty within 12 hours from onset. Diabetes mellitus (DM) was present in 104 of the 327 patients. LV function was serially determined by left ventriculograms taken in the acute and chronic phases (6 months after onset). (I) The early ST-segment resolution rate was lower in DM patients compared with non-DM patients (59% versus 83%, P < 0.0001). (II) During a 6-month follow-up, the percentages of target vessel revascularization (TVR), coronary aorta bypass grafting (CABG), and cardiac death were higher in the DM patients compared with the non-DM patients (TVR: 29% versus 19%; P < 0.05, CABG: 10% versus 5% ; P < 0.05, cardiac death: 12% versus 4%; P = 0.01). (III) The differences in left ventricular ejection fraction (LVEF) between two stages (delta-LVEF) were significantly lower in the DM patients than the non-DM patients (1 +/- 9% versus 7 +/- 10%, P < 0.0001). (IV) Multivariate analysis identified DM as an independent predictor of cardiac death (Odds ratio 5.5, 95% CI, 1.3-23.7, P < 0.05) and as a sole independent predictor of LVEF deterioration (Odds ratio 5.8, 95% CI, 2.8-11.8, P < 0.001). In patients with AMI treated using primary coronary angioplasty, DM is closely related to left-ventricular systolic dysfunction and a poor patient outcome, including mortality.