We studied 25 Japanese patients with hairy cell leukemia (HCL) and found a manner in which HCL can be divided into two subtypes. In each patient, hairy cells (HC) showed striking surface hairs and reacted with HC-specific antibodies (α Leu-M5 and α HC-M). Twenty of the 25 patients had HC characterized by round nuclei with dense nuclear chromatin, weak tartrate-resistant acid phosphatase (TRAP) activity and the phenotype of low density surface immunoglobulin (SIg)⁺, Tac^-. In this group of 20 patients, the male to female ratio was low, and there was frequent leukocytosis. On the other hand, the remaining 5 patients showed a high male to female ratio and a normal or decreased leukocyte count. HC had folded nuclei, strong TRAP activity and the phenotype of high density SIg⁺, Tac⁺. The features of the latter patients are consistent with those of HCL in Western countries, while those of the former group appear to indicate a unique variant of HCL.