Background and objectives:The aim of the present study is to evaluate and compare IgG-binding to megakaryocytes in bone marrow of ITP and MDS patients to determine megakaryocytes targeting by autoantibodies in vivo as a mechanism of platelet underproduction in these disorders. Subjects and methods: The study was carried out on 20 ITP (group I) patients and 20 thrombocytopenic patients with myelodysplastic syndrome (group II) who were admitted to minia university hospital. Serial histological sections from bone marrow biopsies were stained for IgG. Results: high IgG binding was found in ITP and MDS patients (group I & II ) and there was no statistically significant difference between both groups ((14/20 (70%) vs. 13/20 (65%),), (P value =0.736). Conclusion: Antibody binding to megakaryocytes in a proportion of MDS patients suggests that immune mediated mechanism underlies PLT underproduction in those patients