Thrombopoietin (TPO), the primary regulator of platelet production has been identified as megakaryocyte growth and development factor (MGDF). It is produced constitutinely in liver and other organs, circulates in the blood stream, and is delivered to bone marrow, where it stimulates the early development of multiple hematopoietic lineage and megakaryocytopoiesis. It has been proposed that circulating TPO is selectively cleared by platelets through receptor mediated endocytosis and destruction. In this study, we measured serum TPO levels in thrombocytopenic term and preterm babies with sepsis and aimed to investigate the relationship between thrombopoietin levels and platelet counts. We measured serum TPO level using ELISA technique in 59 patients with sepsis and thrombocytopenia and in 11 normal age matched newborn as our control group. All cases and controls were subjected to full history taking, thorough clinical examination, complete blood picture and blood culture. Our results revealed significant relationship between platelet counts and TPO level which is an inverse relationship suggesting that TPO is regulated by a feed back mechanism dependant on the platelet mass. Our data also shows that healthy nonthrombocytopenic term and preterm babies have detectable levels of TPO and preterm babies have higher TPO levels than term infants. The results also show insignificant between sex and platelet count and TPO level. Accordingly more studying and evaluation of TPO regarding its level, pathophysiology, pathogenesis, mode of action, its effect on megakaryocytopoiesis, its recombinant forms and its role in diagnosis, classification, prognosis and treatment of thrombocytopenia is needed.Key Words