Background Polycythemia vera (PV) is a malignant disorder of hemaopoietic stem cells which is characterized by clonal hyperproliferation and a low rate of apoptosis. This study was to assess endogenous erythroid colony (EEC) formation in the bone marrow of PV patients and determine its clinical significance Methods The bone marrow mononuclear cells of 26 patients with PV, 2 patients with secondary erythrocytosis (SE), and 19 normal controls were cultured by Marsh’s method for EEC evaluation, and the clinical significance was evaluated Results EECs appeared in 25 patients with PV but not in 2 patients with SE and 19 normal controls The number of EECs and the EEC ratio [EEC/erythropoietin (EPO)-dependent colony forming unit-erythroid (CFU-E)] in PV patients positively correlated with hemoglobin (Hb) levels Their EEC number did not correlate with white blood cell (WBC) counts, platelet (PLT) counts, or leukocyte alkaline phosphatase (LAP) scores Their EEC did not correlate with serum EPO levels Fifteen patients with PV were treated with hydroxyurea (Hu) and/or interferon-alpha (IFN-α) Their EEC ratio before treatment positively correlated with the treatment time required for complete remission (CR) and negatively correlated with the time before relapse The EEC numbers of 7 PV patients treated with Hu/IFN-α decreased after the blood cell counts dropped to normal levels There was a positive correlation between the EEC ratio and the incidence of attacks of vascular thrombosis in PV patients The numbers of apoptosised bone marrow mononuclear cells in PV patients were lower than those in normal controls The EEC numbers of PV patients negatively correlated with the rate of apoptosis of bone marrow mononuclear cells Conclusions EEC formation is characteristic in PV patients EEC number in PV patients positively correlates with Hb levels, the time required for CR, and the incidence of attacks of vascular thrombosis EEC number negatively correlates with the time before relapse Bone marrow suppressive treatment might decrease EEC number Thus, EEC number is a sensitive and specific parameter reflecting the abnormal hematopoietic clone burden induced by polycythemia vera EEC number is an important diagnostic parameter for PV patients Background Polycythemia vera (PV) is a malignant disorder of hemaopoietic stem cells which is characterized by clonal hyperproliferation and a low rate of apoptosis. This study was to assess the endogenous erythroid colony (EEC) formation in the bone marrow of PV patients and determine its clinical Significance Methods The bone marrow mononuclear cells of 26 patients with PV, 2 patients with secondary erythrocytosis (SE), and 19 normal controls were cultured by Marsh’s method for EEC evaluation, and the clinical significance was evaluated Results EECs were in 25 patients with PV but not in 2 patients with SE and 19 normal controls The number of EECs and the EEC ratio [EEC / erythropoietin (EPO) -dependent colony forming unit-erythroid (CFU-E)] in PV patients positively correlated with hemoglobin (Hb) levels Their EEC number did not correlate with white blood cell (WBC) counts, platelet (PLT) counts, or leukocyte alkaline phosphatase (LAP) scores Their EEC did not correlate wit h serum EPO levels Fifteen patients with PV were treated with hydroxyurea (Hu) and / or interferon-alpha (IFN-α) Their EEC ratio before treatment positively correlated with the treatment time required for complete remission (CR) and negatively correlated with the time The relapse The EEC numbers of 7 PV patients treated with Hu / IFN-alpha decreased after the blood cell counts dropped to normal levels There was a positive correlation between the EEC ratio and the incidence of attacks of vascular thrombosis in PV patients The numbers of apoptosised bone marrow mononuclear cells in PV patients were lower than those in normal controls The EEC numbers of PV patients negatively correlated with the rate of apoptosis of bone marrow mononuclear cells Conclusions EEC formation is characteristic in PV patients EEC number in PV patients positively correlates with Hb levels , the time required for CR, and the incidence of attacks of vascular thrombosis EEC number negatively correlate swith the time before relapse Bone marrow suppressive treatment might decrease EEC number Thus, EEC number is a sensitive and specific parameter reflecting the abnormal hematopoietic clone burden induced by polycythemia vera EEC number is an important diagnostic parameter for PV patients