Hairy cell leukemia (HCL) is an uncommon chronic B-cell lymphoproliferative disorder that is characterized by cytopenia, splenomegaly, and mononuclear cells displaying cytoplasmic projections. We experienced a case of hairy cell leukemia that developed in a 38-year-old man. He showed marked splenomegaly without palpable lymphoadenopathy. A complete blood cell count revealed leukopenia (3300/microliter with 63% of lymphocyte) and the peripheral blood smear showed abnormal lymphoid cells with cytoplasmic projections. The bone marrow smear revealed abnormal lymphocytes and severe myelofibrosis. Tartrate-resistant acid phosphatase reactivity was strongly positive in the hairy cells. The immunophenotyping results of lymphoid cells were CD5 (-), CD10 (-), CD19 (+), CD25 (+), CD103 (+), CD20 (+), lambda (+). The patient was treated with 2-Chlorodeoxyadenosine at a daily dose of 0.1mg/Kg by a continuous intravenous infusion for 7 days. The patient achieved complete remission.