The mutation V617F of gene JAK2 is detected in 95% of patients with genuine polycythemia, in 50% of patients with essential thrombocytemia and idiopathic myelofibrosis. The mutation V617F can be applied as a molecular marker of response to treatment in patients with chronic myeloproliferative diseases associated with this mutation. The technique of quantitative evaluation of V617F (sensitivity up to 0.01%) using polymerase chain reaction is described. This method can be applied to assess the minimal residual disease in patients with chronic myeloproliferative diseases.