A 72-year-old Japanese man presented with 43.1 × 109/l hairy cells and apparent splenomegaly. The leukemia cells had unevenly distributed microvilli and round nuclei with dense chromatin and one or two clear nucleoli, lacked CD25 expression and were negative for tartrate-resistant acid phosphatase. The case was diagnosed as hairy cell leukemia variant (HCLv) and proved refractory to various chemotherapies, including cladribine, pentostatin, interferon-α, CHOP and rituximab. Because of the CD52 expression, we treated the patient with alemtuzumab. Pretreatment with 22.5 Gy to the spleen reduced the spleen size from 12 to 4 cm below the left costal margin, and the number of circulating leukemic cells decreased from 229.0 to 63.6 × 109/l. Subsequent administration of 24.0 mg of alemtuzumab eliminated leukemic cells in the peripheral blood on day 12, and the spleen was not palpable after the administration of 54.0 mg of alemtuzumab. In vitro treatment with alemtuzumab confirmed the cytotoxic effect against the patient’s leukemic cells in the presence of complement. This is the first report showing clinical effectiveness of splenic irradiation and alemtuzumab against refractory HCLv.