We assessed the prognostic significance of negative T waves on admission in leads with ST elevation in 2,853 patients with acute myocardial infarction treated with thrombolysis. Patients were classified into 2 groups based on the presence of negative (T–) or positive (T+) T waves in the leads with ST elevation on admission. T+ and T– waves on admission were detected in 2,601 (91%) and 252 (9%) patients, respectively. T– waves were observed in 6.7 and 9.6% of patients admitted ≤2 and 2–6 h after onset of infarction, respectively. There was a difference in prognosis between patients admitted ≤2 and >2 h after symptom onset. T– patients admitted ≤2 h after onset had no hospital mortality (0/52 patients), as compared to a 5.0% mortality rate in T+ patients (36/726 patients; p = 0.19). T– patients treated >2 h after onset suffered higher mortality (20/196 patients; 10.2%) than T+ patients (100/1,836 patients; 5.4%; p = 0.01). Multivariate analysis of the data on patients treated >2 h after onset demonstrated T– waves to be associated with mortality (OR 1.86; 95% CI 1.07–3.25; p = 0.017). T– waves in leads with ST elevation upon admission are associated with adverse prognosis in patients presenting >2 h after symptom onset, whereas in patients presenting ≤2 h after first symptoms, T– waves may be associated with better prognosis.