X-chromosome DNA probes for the phosphoglycerate kinase (PGK) and hypoxanthine phosphoribosyl transferase (HPRT) genes were used to study clonality in haemopoietic cells from 63 women with myeloproliferative disease, idiopathic erythrocytosis, secondary erythrocytosis or normal red cell volumes. A total of 25 women (39%) were heterozygous for one of the polymorphisms associated with these genes. Clonality was demonstrated in five out of six patients with polycythaemia vera (PV) and in three other patients with myeloproliferative disease. In all cases of PV, including the patient in whom clonality was not demonstrated, cultures of peripheral blood showed growth of endogenous erythroid colonies.