A 75-year-old obese woman with subacute-onset dyspnea and lower right posterior chest pain was brought to the emergency department. She had a history of diastolic heart failure, arthritis, and suspected obstructive sleep apnea. The patient was dyspneic at rest. Pulse oximetry revealed an oxygen saturation percentage in the low 90s while she was receiving supplemental oxygen (4 L by nasal cannula). Trace bilateral peripheral edema was present with no calf swelling. A few crackles were heard at both bases. Cardiac findings were normal, except for a soft ejection systolic murmur at the left lower sternal edge. There was no back or chest wall tenderness. Chest film findings could be interpreted as bibasilar infiltrates or atelectasis. The white blood cell count was 17,000/µL; all other laboratory findings were normal. An ECG showed that the patient was in sinus rhythm with right axis deviation, complete right bundle branch block (RBBB), and anS1Q3T3 pattern; no such abnormalities had been demonstrated on an ECG obtained several weeks earlier. Based on the clinical presentation and the ECG changes, Drs Tapas Bandyopadhyay and Ismael Martin of Farmington, Conn, suspected acute pulmonary embolism. Although the ECG manifestations of pulmonary embolism can vary, findings consistent with right ventricular strain pattern and acute cor pulmonale in the appropriate clinical setting are suggestive. Acute cor pulmonale is demonstrated on an ECG by RBBB, right axis deviation, and an S1Q3T3 pattern. RBBB associated with pulmonary embolism may be transient, or it can persist for up to 3 years after the embolism is detected. In this patient, evidence of segmental mismatch defects on a ventilation-perfusion lung scan confirmed the probability of pulmonary embolism. A venous Doppler study revealed deep venous thrombosis in the right popliteal venous system; an echocardiogram showed a dilated right ventricle with an elevated estimated right ventricular systolic pressure of 50 mm Hg. An intravenous heparin infusion was given; 48 hours later, oral warfarin was initiated. Because the patient was elderly and hemodynamically stable, thrombolytic agents were not used. The patient’s dyspnea and chest pain abated; an ECG showed the RBBB had resolved.