OBJECTIVE: Interferon-alpha (IFNalpha) was the first effective pharmacologic treatment of hairy cell leukemia (HCL). Since 1990 purine analogs replaced IFNalpha because of higher rates of complete remission and an invariable disease recurrence after cessation of IFNalpha. However, there are only limited data about long-term maintenance treatment with IFNalpha and none about dose finding in this phase. PATIENTS AND METHODS: Fifty-two consecutive patients treated at our institution for HCL are included in this retrospective analysis. Forty (77%) patients received IFNalpha and 35 patients continue on long-term IFNalpha maintenance therapy. The initial dose of IFNalpha was 3 Mio IU three times per week and was tapered 6 months after initiation to doses as low as 3 Mio IU/12 wk. Dose adaptation was performed by repeated measurement of soluble Interleukin 2 receptor (sIL2R) together with peripheral blood values. RESULTS: The median follow-up of patients in the long-term IFNalpha group was 13.6 +/- 7.5 yr. Long-term IFNalpha was in general well tolerated and only in six (17%) patients the treatment had to be changed to purine analogs in the long-term IFNalpha group because of side effects. There are no deaths directly related to HCL. CONCLUSIONS: IFNalpha is still an effective and well tolerated therapeutic option. By repeated measurements of sIL2R together with the peripheral blood values, IFNalpha doses can be tapered to the minimal effective dose. The advantages and disadvantage of IFNalpha in regards to the standard treatment in HCL patients are discussed.