MYH9 disorder is characterized by macrothrombocytopenia with or without granulocyte DÖhle body-like inclusion bodies. Diagnosis is made by immunofluorescence analysis and genetic study of the MYH9 gene. Our collaborative study between Thailand and Japan began with 67 Thai patients with macrothrombocytopenia. Of these, 11 patients(16.4%), aged 4 months-22 years with platelet counts ranging from 2,000-99,000/uL were diagnosed with MYH9 disorder. MYH9 gene mutations occurred in exons 1,16,30,38,40. One novel mutation was identified (c.4338T>C, p.F1446A). The results indicate that patients with macrothrombocytopenia should be tested for MYH9 disorder in order to avoid misdiagnosis to the other diseases, such as chronic immune thrombocytopenia.