Chronic Neutrophilic Leukemia (CNL) is a rare myeloproliferative disorder characterized by a persistent increase of mature peripheral neutrophils, myeloid hyperplasia in bone marrow, hepatosplenomegaly, elevated neutrophil alkaline phosphatase (NAP) and absence of Philadelphia chromosome, with no evidence of infection or malignancy sufficient to mimic a leukemoid reaction. CNL has been associated with multiple myelomas in many reported cases, but transition of Polycythemia Vera (PV) to CNL is very rare. An 81-year-old female patient, who had undergone intermittent phlebotomy following the diagnosis of PV 8 years previously, was admitted to our hospital due to lower back pain. A physical examination showed a splenomegaly 2 cm below the costal margin, with tenderness of the thoracic and lumbar spine area. A peripheral blood examination showed a WBC count of 91,800/microL (neutrophil 88%) with a rare immature form, hemoglobin of 9.1 g/dL and a platelet count of 1,661,000/microL. Her NAP score was 58. The bone marrow examination showed 95% cellularity, with an M:E ratio of 10:1, increased megakaryocytes with normal morphology and the absence of myelofibrosis. Chromosomal studies showed no Philadelphia chromosome. A radiological examination showed compression fractures of the vertebrae and spinal cord compression. No underlying disease causing a leukemoid reaction was detected. With iron replacement, the hemoglobin level failed to increase over 12 g/dL. Therefore, it was concluded to be a transition of PV to CNL. After administration of hydroxyurea and vertebroplasty, the symptom improved and the WBC count was sustained below 40,000/microL.