Follow-up data is now available on 144 patients with hairy cell leukemia treated with 2-chlorodeoxyadenosine (2-CdA) at the Scripps Clinic. Of 144 patients followed for a median of 14.2 months, 123 (85%) obtained complete responses, 17 (12%) partial responses, 3 (2%) did not respond and 1 patients was unevaluable. So far, only 4 patients have relapsed at a median of 36 months. Fever was the major toxicity occurring in 43% of patients. Five patients resistant (3 patients) or intolerant (2 patients) to 2'-deoxycoformycin were also treated. Of these 5 patients, 4 obtained complete responses, including 2 patients resistant to 2'-deoxycoformycin, and 1 patient a partial response, suggesting a possible lack of cross-resistance between 2'-deoxycoformycin and 2-CdA in hairy-cell leukemia. More than 200 patients have been treated with 2-CdA worldwide with 82% obtaining complete remission and 12% partial remissions. Serial peripheral blood immunophenotypic analyses have documented the absence of circulating hairy cells. When bone marrow biopsies were examined using sensitive immunohistochemical staining techniques, residual hairy cells were detected in 25-50% of morphologic complete responders. Patients will need to be observed longitudinally to determine if this staining is predictive of relapse. A double-blind, placebo-controlled study of pentoxifylline, a modulator of tumor necrosis factor-alpha and other cytokines, was performed in 2-CdA-treated hairy cell leukemia patients to determine whether the incidence of neutropenic fever would be reduced. Although pentoxifylline resulted in less febrile, hospital and antibiotic therapy days than placebo, only the number of days in the hospitalized patients achieved statistical significance. 2-CdA is emerging as the treatment of choice for hairy cell leukemia given the high percentage of long-lasting, complete responses that follow a single course of therapy.