Hydroxyurea treatment reduces haematopoietic progenitor growth and CD34 positive cells in polycythaemia vera and essential thrombocythaemia. 
The aim of the present work was to investigate the effect of hydroxyurea (HU) treatment on haematopoietic progenitors and CD34 positive (CD34+) cells in patients with polycythaemia vera (PV) and essential thrombocythaemia (ET).  Of the PV patients were 10 treated with phlebotomy only and 10 were on HU therapy.  Seven ET patients were untreated and 10 received HU.  In each subject peripheral blood was obtained for in vitro colony growth, determination of CD34+ cells and plasma erythropoietin (EPO) concentration.  The mean number of EPO independent erythroid colonies (EEC) was higher in the group of PV patients on phlebotomy therapy compared to the PV patients treated with HU (74.4 and 8.0 colonies/10(5) cells, respectively) but the difference did not reach statistical significance.  The corresponding means for the untreated ET patients and ET patients treated with HU were 13.0 and 1.3 colonies/10(5) cells, respectively, this difference being statistically significant (p = 0.012).  The mean EEC for combined groups of PV and ET without myelosuppressive treatment were compared with the results for PV and ET patients on HU therapy; this difference was statistically significant (p = 0.014).  The same pattern was observed for total erythroid growth with EPO.  The relationship between the concentration of CD34+ cells and total EEC in peripheral blood was statistically significant for both PV (p<0.005) and ET (p<0.01).  This finding supports the hypothesis that the level of CD34+ cells in peripheral blood could be used as a proliferation marker in these two myeloproliferative entities.  No relationship between plasma EPO and EEC was present.  It therefore appears that the reported differences in plasma/serum EPO concentrations between PV patients on phlebotomy treatment compared to patients on myelosuppressive treatment are not likely to be found at the production site for erythrocytes.