A 67-year-old male was admitted because of lymphocytosis and huge splenomegaly. Abnormal lymphocytes had cytoplasmic hairy projections and were negative for tartrate-resistant acid phosphatase staining. The bone marrow aspirate contained many lymphocytes with the same morphology. Flow cytometric analysis revealed an increase in IgM and κ positive B cells. They were positive for CD11c, CD19, CD20 and FMC7, and negative for CD5, CD10 and CD25. The patient was diagnosed as having hairy cell leukemia, Japanese variant. Initially interferon-α was administered for a month, decreasing the numbers of leukemic cells but with little effect on splenomegaly. Subsequent administration of cladribine (0.09 mg/kg, 7 days) showed a remarkable effect, and the patient has been in complete remission for 8 months.
