Chronic thromboembolic pulmonary hypertension (CTEPH) is characterized by a fibrotic thrombus persisting and obliterating the lumen of pulmonary arteries; its pathogenesis remains poorly defined. This study investigates a potential contribution for progenitor or primitive cell types in the development of vascular obliteration and remodeling in CTEPH patients. Endarterectomized tissue from patients undergoing pulmonary thromboendarterectomy (PTE) was collected and examined for the structure and cellular composition. Our data show an organized fibrin network structure in unresolved thromboemboli and intimal remodeling in vascular wall tissues, characterized by SM--actin⁺ cell proliferation in proximal region (adjacent to thromboemboli) and neo-angiogenesis/recanalization in distal region (downstream from thromboemboli). Cells that are positively stained with CD34 and Flk-1 (CD34⁺Flk-1⁺) were identified in both the proximal and distal vascular tissues; a subpopulation of CD34⁺Flk-1⁺CD133⁺ cells were further identified by immunostaining. Triple positive cells are indicative of a population of outgrowth endothelial progenitor cells or colony forming units of endothelial cells (CFU-EC). In addition, inflammatory cells (CD45⁺) and collagen secreting cells (procollagen1⁺) were detected in the proximal vascular wall. Some of the CD34⁺ cells in CTEPH cells isolated from proximal regions were also positive for SM--actin. Our data indicate that primitive cell types are present in the neointima of occluded vessels of CTEPH patients. It is possible that the microenvironment provided by thromboemboli may promote these primitive cells (and the vascular resident progenitor cells) to differentiate and enhance intimal remodeling.