To evaluate the time course of reversed remodeling after pulmonary endarterectomy (PEA) in patients with chronic thromboembolic pulmonary hypertension(CTPEH), we studied 22 patients (age: 60 +/- 13 years) with MRI immediately before, 1 month, 3 months, and 6 months after PEA. MRI included assessment of biventricular function, aortic and pulmonary artery(PA) flow, and right ventricular (RV) overload using the ratio of RV-to-biventricular diameter. Except in one patient, who died 2 months post-surgery, clinical improvement occurred early after PEA (NYHA class: 3.3 +/- 0.6 to 1.5 +/- 0.8, p < 0.0001) with a decrease of systolic pulmonary artery pressures (79 +/- 14 to 44 +/- 14 mmHg, p < 0.0001). At 1 month post PEA, RV end-diastolic volumes decreased (198 +/- 72 to 137 +/- 59 ml, p < 0.0001), and the RV ejection fraction (EF) improved (31 +/- 9 to 47 +/- 10%, p < 0.0001). No further significant improvement in pulmonary pressures or RV function occurred at 3 months or 6 months. Although no significant change was found in LV volumes or function, aortic flow increased early after surgery. PEA had only a beneficial effect on right PA flow. RV overload decreased early after PEA (ratio RV-to-biventricular diameter: before: 0.67 +/- 0.04, after: 0.54 +/- 0.06, p < 0.0001), showing a good correlation with the improvement in RVEF (r = 0.7, P < 0.0001). In conclusion, reversed cardiac remodeling occurs early after PEA, to slow down after 1 month. At 6 months, cardiac remodeling is incomplete as witnessed by low-normal RV function and residually elevated PA pressures.