To determine whether an increased circulating CD34-positive cell count is associated with evolution to secondary myelofibrosis, we compared the absolute values and relative variations for CD34-positive cell counts in patients with PV or ET at the time of progression to myelofibrosis (n = 29) and patients without haematological evolution at the latest follow-up (n = 104). Here, CD34 is linked to acquired polycythemia vera.