Our aim was to study the cause and describe the clinical features of pulmonary arterial hypertension (PHT) in splenectomized β-thalassemia (β-Thal) patients. Ten splenectomized β-Thal patients with systolic pulmonary artery (PA) pressure >30 mm Hg were evaluated by echocardiography, right-heart catheterization, and pulmonary angiography. Five of these patients later underwent hemodynamic studies. Echocardiography and pulmonary angiography on the 10 patients showed normal values of left ventricular systolic function and no findings of acute or chronic pulmonary embolism. Hemodynamic evaluation showed very high PA pressures associated with markedly increased pulmonary vascular resistance indices (PVRIs). Hematological evaluation of the 10 patients showed marked anemia, markedly increased numbers of nucleated red blood cells (nRBCs), and serum ferritin. Mean platelet count, plasma β₂ thromboglobulin, and thrombin—antithrombin III complex levels were significantly increased. It was concluded that PHT can be found in splenectomized β-Thal patients. Features associated with PHT were female sex, hemoglobin E/β-Thal, status many years postsplenectomy, marked anemia, markedly increased nRBC count, thrombocytosis, and very high serum ferritin levels. PHT was not due to pulmonary emboli. Our findings suggested that severe PHT was due to increased PVRI from thrombotic pulmonary arteriopathy, likely from chronic low-grade hypercoagulability and platelet activation after splenectomy.