Although thrombocytosis has long been recognized as a common finding in children with hepatoblastoma, its mechanism is still unknown. In this study, to confirm the role of thrombopoietin (Tpo) in the thrombocytosis in hepatoblastoma, the expression of Tpo mRNA in tumor cells was examined and the serum Tpo level was analyzed during the course of the disease. A 1-year, 6-month-old girl was diagnosed as having advanced hepatoblastoma. At diagnosis, she had marked thrombocytosis with 1220 x 10(9)/microL. After resection of the tumor and after four courses of chemotherapy, the level of alpha-fetoprotein normalized but the platelet count remained high. However, after the fifth course of chemotherapy, the platelet count decreased and normalized within 3 months. By enzyme-linked immunosorbent assay, the serum Tpo level was not high at diagnosis, whereas high Tpo levels were observed after chemotherapy. By polymerase chain reaction, Tpo mRNA was detected in both normal liver and tumor tissues, but the level of expression was not different between them. Therefore, in hepatoblastoma the serum Tpo level is not correlated with a high platelet count, and there is no difference in Tpo expression between normal liver and tumor tissues. Other unknown factors and their production sites to induce thrombocytosis should be examined in further studies.