Thrombopoietin (TPO) is thought to be the major physiological regulator of thrombopoiesis, and, in general, circulating levels are inversely proportional to megakaryocyte and platelet mass. However, normal or elevated TPO levels are found in patients with essential thrombocythaemia (ET) and the reason for this is not fully understood. Recent studies have shown that four kindreds with hereditary thrombocythaemia (HT) have point mutations in the 5'-untranslated region (UTR) of the TPO gene which lead to increased TPO translation. In order to determine whether similar mutations are present in apparently acquired ET, in particular in those patients with polyclonal myelopoiesis, we have studied this region in 50 ET patients using neutrophil DNA. The known HT mutations were investigated using polymerase chain reaction with mismatch primers and restriction enzyme digestion; only wild-type alleles were detected. Single-stranded conformation polymorphism (SSCP) analysis of exons 1-4 identified a C-->T substitution at nucleotide 3767. However, this appears to be a common polymorphism, as it was present at the same frequency in haematologically normal controls and is unlikely to be of pathological significance. These results demonstrate that mutations in the 5' UTR of the TPO gene are not the cause of the normal or elevated TPO levels in acquired ET.