Hairy cell leukemia is a rare chronic B-lymphocyte malignancy characterized by pancytopenia, splenomegaly, immunological abnormalities and morphologically typical neoplastic mononuclear cells in blood and bone marrow. Until recently the disorder was treated with splenectomia and/or alpha-interferon. Intravenous infusion of 2-chloro-deoxy-adenosine (2-CdA) is a new and efficacious treatment principle making hairy cell leukemia a potentially curable disease. A Nordic cooperative study is currently examining the effects of 2-CdA given subcutaneously for seven days. 19 Norwegian patients are included so far, with encouraging results. The long-term effect of 2-CdA is not yet known. The high remission frequency, the short duration of the treatment and the few side effects make this drug a natural first choice in treating hairy cell leukemia.