Essential thrombocythaemia (ET) is a condition of unknown aetiology characterized by sustained thrombocytosis in the absence of a detectable systemic cause. Although usually considered a clonal disease affecting myeloid cells, recent data indicate that a significant proportion of patients have polyclonal haemopoiesis. In some patients the thrombopoietin (TPO) levels are normal or raised. Recently a mutation has been described in the TPO gene in familial thrombocythaemia that results in elevated TPO levels. We have therefore screened 51 patients diagnosed with non-familial ET for the presence of this mutation, but it was not detected in any patient. The constitutional presence of this mutation is therefore unlikely to contribute to the pathogenesis of ET.