Essential thrombocythemia is a myeloproliferative disorder not known to have consistent cytogenetic abnormalities. A 38-year-old woman with essential thrombocythemia having Philadelphia chromosome (Ph1) is reported. The patient first presented with gastrointestinal bleeding, accompanied by thrombocytosis. Treatment of the gastrointestinal bleeding did not influence the elevation of platelet counts. The patient's clinical and hematological manifestations were consistent with essential thrombocythemia, but not with any other myeloproliferative diseases. Ph1 chromosome was constantly proved in bone marrow preparations from this patient over two years and four months and gave us a certain impression that Ph1 chromosome might have had some relation to development of essential thrombocythemia to chronic myelogenous leukemia in this patient.