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Summary: A persistently elevated platelet count is seen occasionally as a presumed neoplastic phenomenon in polycythaemia vera, myelofibrosis and chronic granulocytic leukaemia, all variants of the myeloproliferative syndrome. More rarely a group of cases is seen where the elevated platelet count appears the dominant finding and the condition is variously called idiopathic or essential thrombocythaemia, haemorrhagic thrombocythaemia, megakaryocytic myelosis or a variety of other synonyms. The clinical, haematological and cytogenetic features of seven cases in Western Australia over a ten year period have been reviewed. The clinical picture was dominated by haemorrhage in three patients and thromboembolic phenomena occurred in four, on occasion concurrently with bleeding. Splenomegaly was present in all cases. No patient manifested erythrocytosis either at presentation or on follow‐up. All had peripheral blood platelet counts persistently over 800,000/μl and megakaryocytic hyperplasia was the principle finding on bone marrow examination. Where performed tests of platelet function were normal. Chromosome analysis was carried out in all patients; direct bone marrow preparations were studied in five patients and peripheral blood cultures in two patients. There was a high degree of aneuploidy and a high polyploidy index. One case was found to have a marker chromosome. None had any G group chromosome abnormality. With adequate control of the platelet count using 32P or busulphan, the prognosis appears to be good. These patients have survived for a mean of 7.5 years from the onset of symptoms (but only a mean of 3.0 years from establishment of a definite diagnosis). One patient has died from termination as acute leukaemia, a rare event in this condition. It is concluded that the features found in these patients are distinctive enough to warrant consideration of idiopathicthrombocythaemia as a distinct nosological entity in the spectrum of the myeloproliferative disorders.