Three patients with advanced hairy cell leukemia received low-dose deoxycoformycin treatment after failure to respond to therapy with interferon alpha. Patients 1 and 2 had progressive disease after splenectomy and subsequent treatment with recombinant interferon alpha (for 7 and 3 months, respectively). DCF was administered at 4 mg/m2 weekly for 3 weeks, and then once every week for 6 weeks. Patient 1 was in complete remission after 9 weeks of treatment and patient 2 in partial remission with normalization of peripheral blood counts. The third patient, also splenectomized, developed hepatotoxicity after therapy trial with interferon for 24 days and no objective improvement was observed at this stage. She subsequently responded to DCF treatment with improvements in blood counts and bone marrow. This report demonstrates that DCF is highly effective in hairy cell leukemia and non-cross-resistant with interferon alpha.