We investigated the role of the standard 12-lead electrocardiogram (ECG) to improve the pretest probability of pulmonary embolism before performing computed tomographic (CT) pulmonary angiography. A retrospective chart analysis was performed on patients who underwent CT pulmonary angiography at a tertiary care hospital during a 30-month period. Comparison of 15 ECG parameters was made between those with CT pulmonary angiograms positive for pulmonary embolism and a matched control group with negative CT pulmonary angiograms. Data were analyzed by chi-squared tests and logistic regression. Sinus tachycardia (39% vs. 24%, P <0.01), an S1 Q3 T3 pattern (12% vs. 3%, P <0.01), atrial tachyarrhythmias (15% vs. 4%, P <0.005), a Q wave in lead III (40% vs. 26%, P <0.02), and a Q3 T3 pattern (8% vs. 1%, P <0.02) were the findings significantly associated with pulmonary embolism. We conclude that 1) standard 12-lead ECG findings can increase the pretest probability of pulmonary embolism before performing CT pulmonary angiography; and that 2) the ECG findings have relatively low likelihood ratios to have clinical use.