OBJECTIVE: There are conflicting results regarding the impact of type 2 diabetes on intravenous thrombolysis effectiveness during ST elevation myocardial infarction (STEMI). The present study, using a continuous 12-lead electrocardiogram, examined the possible association of type 2 diabetes with both acute intravenous thrombolysis effectiveness and long-term prognosis in this setting. RESEARCH DESIGN AND METHODS: The study included 726 consecutive subjects (214 type 2 diabetic subjects) with STEMI who received intravenous thrombolysis in the first 6 h from index pain and were followed up for 3.5 years. RESULTS: Type 2 diabetic subjects had significantly lower incidence of sustained > or = 50% ST recovery than nondiabetic subjects (P = 0.03). Additionally, the former required a significantly greater time interval through the achievement of this criterion than the latter (P < 0.001). In both type 2 diabetic (P < 0.001) and nondiabetic subjects (P < 0.001), those who had not attained > or = 50% ST recovery were at significantly higher risk of cardiac death than subjects who had reached this criterion. The subjects who attained the above electrocardiographic criterion in > or = 60 min after thrombolysis initiation were at significantly higher risk compared with those who achieved this criterion in <60 min (P = 0.02). However, this association was true only for type 2 diabetic subjects (P = 0.01) and not for nondiabetic subjects (P = 0.9). CONCLUSIONS: The present study suggests that type 2 diabetes is a strong predictor of acute intravenous thrombolysis failure during STEMI. This finding may significantly contribute to the worse prognosis for type 2 diabetic subjects compared with nondiabetic ones in this setting.