The pathogenesis of the increased number of megakaryocytes and thrombocytosis in essential thrombocythemia (ET) is still unknown. We examined the expression of c-mpl, a receptor of thrombopoietin (TPO), and its signaling molecules in a patient with ET. An 8-year-old girl showed a high platelet count and an increased number of bone marrow megakaryocytes. Neither chromosomal abnormalities nor myelofibrosis was observed. Following the diagnosis of ET, aspirin therapy was begun for the patient, with only modest improvement of symptoms. Her platelet count ranged from 1,200,000/microL to 2,200,000/microL for more than 2 years. In the analyses, the serum TPO level in the patient was 420 attomoles/mL (normal, 760 +/- 320). The level of c-mpl expression in bone marrow mononuclear cells was higher in the patient than in healthy children, while there was no difference in the level of c-mpl expression in CD34+ cells, indicating an expanded pool of megakaryocytic lineage cells. The level of Janus kinase 2 (Jak2) expression was lower in the patient than in a healthy child. These findings indicate that the signal pathway mediated by c-Mpl after binding to TPO may be impaired in ET. Further analysis is needed to clarify the mechanism underlying the development of thrombocytosis in ET patients.