Introduction Myosin heavy chain-9(MYH9) related disorders such as May-Hegglin anomaly(MHA) are rare AD disorders characterized by thrombocytopenia with giant platelets(megathrombocytopenia) and basophilic cytoplasmic inclusion bodies(resembling Döhle bodies) in the leukocytes. MHA manifest mild bleeding tendency and may develop kidney dysfunction, deafness and cataracts later. MHA do not require treatment, but MHA may be misdiagnosed as acute ITP and treated. We report a case of MHA diagnosed at 12 years of age and followed up until 23 years of age. CASE An 11-year-old girl was admitted to our hospital due to frequent epistaxis and menorrhagia, and severe thrombocytopenia with platelet count of 6,000/μL in 2003. She had been diagnosed as ITP and treated with high dose IVIG therapy twice without response in another hospital. She did not have family history of coagulopathy. Her other laboratory data including PT/aPTT, BT, bone marrow biopsy were normal. The patient went to America for overseas studies, and she was diagnosed as MHA at the Philadelphia Children\'s Hospital with observation of giant platelets in PB morphology. She came back to Korea, and had been regularly followed up at the outpatient clinic of our Pediatric Department without medication. Her platelet count was in the range of 6,000~20,000/μL. During the past 11 years, she lived usual life with frequent epistaxis, but without other severe bleeding symptoms. She was treated with iron for 3 months due to iron deficiency anemia. At 23 years of age, she was again admitted to our hospital for laboratory tests follow up and transfusion of platelet for 3rd molar tooth extraction. CBC showed Hb12.6g/dL, Hct37.6%. WBC2,800/μL with ANC of 1456, and platelet count of 12,000/μL. Her PB smear showed absolute neutropenia and giant platelets. Rapid plasma aggregating test was prolonged. PNH study was negative. Bone marrow study showed increased granulopoiesis and megakaryopoiesis. Other laboratory data, including electrolyte, liver enzyme, BUN/creatinine, PT, aPTT, vWF antigen and activity, and plasma mixing test were normal. We performed mutational analysis using PCR and direct DNA sequencing of the MYH9 gene. Her platelet count increased to 60,000/μL and rapid plasma aggregating test normalized after platelet transfusion. 3rd molar tooth extraction was performed without bleeding complication.