Nine patients with progressive hairy cell leukemia were treated with subcutaneous injections of recombinant alpha 2 interferon (2 to 10 X 10(6) U/m2) three times weekly. Eight patients completed at least eight weeks of treatment and were evaluable; one patient with refractory thrombocytopenia died of an intracerebral hemorrhage after two doses of interferon. Seven of eight patients responded, with responses occurring as early as two weeks. Four patients also had resolution of their monocytopenia. No complete responses were seen with up to 30 weeks of treatment. Bone marrow biopsies demonstrated improvement in all eight patients. No unforeseen toxicity occurred, but most patients had transient myelosuppression during the first few weeks of treatment. Recombinant alpha 2 interferon is effective in the treatment of hairy cell leukemia, with acceptable toxicity.