The authors describe a case of thrombocythaemia, with subsequent leukaemic transformation. Cytochemical and immunocytochemical investigations indicated a trilineage involvement of the myeloid series, compatible with a leukaemic transformation at the level of the colony-forming unit granulocytes, erythrocytes, macrophages, megakaryocytes. No cytogenetic abnormalities were observed. The criteria which have been proposed to differentiate essential thrombocythaemia from pre-fibrotic thrombocythaemia, as an early phase of idiopathic myelofibrosis, are discussed. The differentiation is not only of academic interest but has relevant practical implications, since survival in the two conditions is significantly different. The possible significance of an accompanying monoclonal gammopathy is discussed.