We studied the clinical and laboratory characteristics of 71 patients with hairy cell leukemia to ascertain whether prognostic factors important for the therapy of the disease existed. Granulocytopenia of less than 500/µ1 was associated with the occurrence of a serious infection in 46% of the patients, compared to 19% of patients whose absolute granulocyte count was greater than 500/µ1 (P < 0.05). In most patients with symptoms of thrombocytopenia or leukopenia, splenectomy led to improved blood counts. Patients treated with splenectomy only had a significantly longer survival than those treated with steroids only (P < 0.05). A group of patients who were slightly older and had minimal splenomegaly and few hairy cells in the peripheral blood did well without therapeutic intervention. Twenty-four patients have died, 16 of an infectious process. The actuarial survival for the group is 51% at 4 years, and there has been no change subsequently.