Background: Aggressive management such as thrombolytic therapy is necessary to save life in patients with acute severe pulmonary thromboembolism (PTE). ECG changes in association with PTE are usually nonspecific but severe PTE can produce some specific ECG findings. The purpose of this study is to determine the role of 12-lead ECG in the evaluation of the severity of acute PTE to help therapeutic decision making. Methods: We analysed 42 patients (60.5짹14.9 years, 17 men) in whom PTE was confirmed by chest CT angiography and 63 control patients (60.3짹16.5 years, 21 men) who had various cardiovascular diseases. Systolic pulmonary artery pressure (sPAP) was estimated by Doppler echocardiography (4 * Tricuspid valve flow velocity짼 + 10 mmHg). Results: In 12-lead ECG, sinus tachycardia (35.7% vs 12.7%, p = 0.005), QRS axis over 90 degree (28.5% vs 7.9%, p = 0.005), S1Q3T3 sign (45.2% vs 23.8%, p = 0.021), T-wave inversion in V2-4 (31.0% vs 14.3%, p = 0.040) were more common in the PTE group than in the control group. There was no significant ECG change in 3 patients (7.1%) in the PTE group. In the PTE group, T-wave amplitude in the leads II, III, aVF showed correlation to sPAP (r = -0.400, p = 0.009; r = -0.289, p = 0.063; r = - 0.321, p = 0.038, respectively). T-wave amplitude in lead II of ��� 0.5 mm predicted severe pulmonary hypertension (��� 50 mmHg) in patients with PTE with a sensitivity of 59.3%, a specificity of 86.7%, a positive predictive value of 88.9%, and a negative predictive value of 54.2% (OR = 6.532, 95% CI, 1.592 to 26.785). Conclusions: T-wave amplitude (cut-off value = 0.5 mm) in the ECG lead II is very simple and useful to distinguish the patients with from without severe pulmonary hypertension (��� 50 mmHg) and to evaluate the therapeutic response.