The hairy cell leukemia prolymphocytic variant, a subtype of hairy cell leukemia, is an extremely rare disease, especially in Japan. We report a case in which treatment with 2'-deoxycoformycin (DCF) improved the clinical features of the disease. The patient, a 70-year-old female, was first treated with 2-chlorodeoxyadenosine, but showed only transient improvement in the hematological findings. DCF was then administered every week. Following the start of this treatment, the leukemia cell count rapidly decreased and the platelet count simultaneously increased. This effect of DCF has so far been long term. More clinical studies are needed to confirm the therapeutic value of DCF.