Non-Hodgkln's lymphoma cell leukemia (NHLCL), chronic lymphocytic leukemia (CLL) and hairy cell leukemia (HLC) are the diseases very similar to each other. The differential diagnosis is very difficult, especially when there are small lymphoid cells in peripheral blood and bone marrow under light microscope. We have observed 34 cases with electron microscope. The studies were correlated with clinical manifestation, cytology, pathology and immunologic histochemistry. Ultrastructural features strongly indicated the difference in three various diseases, although all the immunologic markers showed B-cell type. It is concluded that electron microscopic examination is of a definite significance in the diagnosis and successful treatment.