Essential thrombocythaemia (ET) is a myeloproliferative disease (MPD) predominantly involving the platelet lineage, with a diagnosis often difficult to establish in practice. The demonstration of a clonal haemopoiesis can contribute to diagnosis. The clonal origin of blood cells can be assessed in female patients using X-chromosome-linked polymorphisms, assuming a random inactivation of the X chromosome. The human androgen receptor gene HUMARA has been used for this purpose, taking advantage of a highly polymorphic trinucleotide repeat in the first exon. The close proximity of the polymorphic trinucleotide repeat to four methylation sites permits a clonal analysis based on the polymerase chain reaction. We have used this technique to study the clonality of haemopoietic cells in 32 patients with ET and 23 age-matched control heterozygotes for the HUMARA polymorphism. A monoclonal pattern of haemo-poiesis was detected in unfractionated nucleated blood cells from 22 patients, suggesting that haemopoiesis is essentially monoclonal in a majority of cases in this disease. In some patients a monoclonality of granulocytes was documented, whereas the mononuclear fraction had a polyclonal pattern of X-inactivation. Finally, in two patients for whom a polyclonality had been found in unfractionated blood, analysis of G6PD transcripts in platelets revealed a clonal megakaryocytopoiesis. These findings confirm the heterogeneity of haematological abnormalities in ET patients and the potential utility of a multifaceted laboratory approach to investigate these patients.