Chronic thromboembolic pulmonary hypertension (CTEPH) is associated with abnormal left ventricular (LV) filling hemodynamics [mitral early passive filling wave velocity/late active filling wave velocity (E/A) < 1]. Pulmonary endarterectomy (PEA) acutely reduces pulmonary vascular resistance, resulting in an increase of mitral E/A. The abolishment of leftward septal bulging and an increase in right ventricular (RV) output are thought to be responsible for the increase of mitral E/A. In this study, we quantified the separate effects of leftward septal bulging and RV output on LV hemodynamics. In 39 CTEPH patients who underwent PEA, transmitral flow velocities and RV hemodynamic data were obtained pre- and postoperatively. A mathematical model describing the mechanics of ventricular interaction was fitted to the preoperative average values of cardiac output (CO; 4.4 l/min), mean pulmonary artery pressure (mPAP; 50 mmHg), mitral E/A (0.74), and mean left atrial pressure (mLAP; 9.8 mmHg). Starting from this preoperative reference state with leftward septal bulging, PEA was simulated by changing mPAP and CO to average postoperative values (28 mmHg and 5.7 l/min, respectively). Simulated and postoperatively measured data on E/A (1.27 vs. 1.48), mLAP (12.6 vs. 11.5 mmHg), and septal curvature (both rightward) were consistent. When an exclusive decrease of mPAP was simulated, mitral E/A increased 26%, mLAP decreased 16%, and septal curvature became rightward. When an exclusive increase of CO was simulated, mitral E/A increased 53% and mLAP increased 62%, whereas leftward septal bulging persisted. Thus, our simulations suggest that the increase of mitral E/A with PEA is caused two-thirds by an increase of RV output and one-third by the abolishment of leftward septal bulging.