Hairy cell leukemia (HCL) is a chronic lympho-proliferative disease. The disease is clinically characterized by anemia, granulocytopenia, thrombocytopenia, monocytopenia, splenomegaly. This leukemia cells have peculiar shape with hairlike projection, and express mature B-cell antigens such as, CD19,CD20 and Ig heavy chain on their surface. Nuclear and cytoplasm area ratio was low. Cell biological observation and chromosome analysis were performed in 12 patients. Translocation at 14q32 was found in 2 patients and loss of chromosome 6 was found in one patient. These cytogenetical results suggest taht HCL is different disease from chronic lymphocytic leukemia.