Background: Circulating endothelial cells (CECs), originated form endothelial progenitors (EPCs) are mature cells not associated with vessel walls and detached from the endothelium. Normally, they are present in insignificant amounts in the peripheral blood of healthy individuals. On the other hand, elevated CECs and EPCs levels have been reported in the peripheral blood of patients with different types of cancers and other diseases. Objective: This review aims to provide an overview on the characterization of CECs and EPCs, to describe isolation methods and to identify the potential role of these cells in hematological diseases and hematopoietic stem cell transplantation. Methods: We performed a detailed search of peer-reviewed literature using keywords related to CECs, EPCs, allogeneic hematopoietic stem cell transplantation, and hematological diseases (hemoglobinopathies, hodgkin and non-hodgkin lymphoma, acute leukemia, myeloproliferative syndromes, chronic lymphocytic leukemia). Results: CECs and EPCs are potential biomarkers for several clinical conditions involving endothelial turnover and remodeling, such as in hematological diseases. These cells may be involved in disease progression and in the neoplastic process. Moreover, CECs and EPCs are probably involved in endothelial damage which is a marker of several complications following allogeneic hematopoietic stem cell transplantation. Conclusion: This review provides information about the role of CECs and EPCs in hematological malignancies and shows their implication in predicting disease activity as well as improving HSCT outcomes.