We report a rare case of chronic neutrophilic leukemia (CNL) which terminated in acute myeloblastic transformation 3 years after the onset of the disease. The increased leukocytes were mainly neutrophils at various maturational stages until 1 month before transformation without dysplastic hematopoietic cells or other myeloproliferative disorders. Repeated analyses for the Philadelphia chromosome (Ph1), rearrangement of the BCR gene or chimeric BCR/ABL mRNA, major, minor and mu, were negative. Genomic analysis of granulocyte colony-stimulating factor (G-CSF) receptor did not reveal any abnormality. The clinical manifestations were characterized by hyperleukocyte syndrome with respiratory distress and ischemic legs with gangrene.