Early age onset of hairy cell leukemia presenting with leukocytosis

Hairy cell leukemia (HCL) is an uncommon type of chronic B cell leukemia mainly affecting middle-aged adults. HCL presenting with pancytopenia is rare in Japan and a distinct subtype of HCL termed HCL-Japanese variant is predominantly seen. We describe a HCL patient with unusual presentation. The patient was a 26-year-old male, such early onset of HCL being quite rare. The patient showed leukocytosis with many circulating hairy cells and cellular bone marrow. These findings were preferentially seen in HCL-Japanese variant, but, cytomorphologic, cytochemical and immunophenotypical studies on the pathologic cells were consistent with those of typical HCL seen in Western countries. Interferon-alpha therapy was very effective in this case. Differentiation of the subtype of HCL appears to be important for the choice of the treatment. The cytological findings were useful for the differential diagnosis of HCL presenting with leukocytosis.