Primary polycythaemia (PP), idiopathic myelofibrosis (MF), essential thrombocythaemia (ET) and chronic granulocytic or myeloid leukaemia (CGL) are clonal disorders of the pluripotent haemopoietic stem cells. We have studied granulocyte, megakaryocyte and erythroid progenitors from the peripheral blood of 7 patients with PP, 9 with ET, 19 with MF and 6 with CGL in order to characterise similarities and differences at the committed progenitor cell level. Spontaneous megakaryocytic and erythrocytic growth was characteristic of MF, PP and ET but was not seen in CGL. Circulating erythroid (BFU-E) and granulocyte/macrophage (CFU-GM) progenitors were markedly increased in MF and CGL, less raised in ET and closest to normal in PP. Erythropoietin-independent erythroid bursts (EIBFU-E) grew from the blood of patients with MF, PP and ET but spontaneous growth of megakaryocytes occurred in only MF and ET. These results suggest a progression of increasing abnormality from PP, where EIBFU-E occurred with relatively normal numbers of circulating progenitors, to ET where both EIBFU-E and megakaryocyte precursors regularly occur with elevated numbers of progenitors, to MF where spontaneous BFU-E, CFU-Mk and CFU-GM occur at high levels.