Basing on 13 cases of hair cell leukemia the gross morphology, histopathology and electron microscopy of this peculiar neoplasis is presented. The tricholeukocytes, as seen in Giemsa's preparation and especially in phase and electron microscope are pathognomic for establishing the diagnosis. In some cases the hairy cells may also be recognized in a HE stained tissue section. The staining method with Amidoblack 10 B may be useful for demonstrating hairy surface of the losely arranged neoplastic cells. Using a method with reduced Protargol and especially Timm's reaction after alcohol-sulfide fixation, a strong positivity could also be noticed. Exceptionaly the surface of free tricholeukocytes was found to be coated by proteinaceous precipitate resembling feature in Hoeppli's phenomen.