Abstract: Thrombopoietin (TPO) is a cytokine that has been isolated and identified as the primary growth factor responsible for regulation of megakaryopoiesis and thrombopoiesis. The aim of this work was to study thrombopoietin (TPO) level in normal adults and children as well as its level estimation in quantitative platelet disorders. The correlation between TPO levels and platelet count was also studied. The correlation between TPO levels and ultra-structure changes of platelets was studied as well. The present study was conducted on 100 persons. They were 40 patients selected from the outpatient clinic of National Cancer Institute, Cairo University and they had quantitative platelet disorders in the form of thrombocytopenia and thrombocytosis. The remaining 60 cases were apparently healthy individuals Assessment of TPO assay to all the studied groups was done by using ELISA method. Examination by Transmission Electron Microscopy (TEM) has been done for the circulating blood platelets of selected cases of quantitative platelet disorders as well as for some normal control cases. Significant differences were found between TPO levels in children and neonates groups as compared to normal adult control. Highly significant inverse correlation between TPO and platelet counts in adult control group. Comparison between TPO means was done in three subgroups as regards megakaryocyte frequency (normal MK1, decreased MK 2, increased MK 3) by using ANOVA test. ANOVA revealed significant statistical difference between TPO means of normal (MK1, n =19), TPO means of decreased megakaryocyte frequency (MK2, n = 10) and TPO means of increased megakaryocyte frequency (MK3, n =11). It was observed that high values of TPO levels were accompanied by decreased megakaryocyte frequency and lower TPO values accompanied by increased megakaryocyte frequency. Therefore, TPO levels are inversely related to megakaryocyte frequency. The ultrastructure study of circulating blood platelets was done for some selected and representative cases from thrombocytopenia group and thrombocytosis group as well as normal control group. It was observed that normal platelet morphology seen by E.M. was accompanied by normal TPO levels. While, abnormalities seen in platelets of thrombocytopenia were accompanied by abnormal high TPO levels. In thrombocytosis cases, abnormalities seen in platelets were accompanied by relatively lower TPO levels than that of normal cases.