Sixty-nine patients with hairy-cell leukemia (HCL) were treated with interferon alfa-2b (IFN) in a single-institution study. The dose used was 2 x 10(6) U/m2 self-administered subcutaneously three times weekly, for a planned treatment duration of 12 to 18 months. Of the 68 evaluable patients, the major response rate was 75%, with 13% complete responses (CRs) and 62% partial responses (PRs). An additional eleven patients (16%) had minor responses (MRs). Duration of response was denoted as failure-free survival (FFS), defined as the time from the end of IFN therapy to a need for further antileukemic therapy. Of the 60 responding patients followed after discontinuation of IFN, 27 have relapsed, requiring further therapy. The median actuarial FFS for these 60 patients is 25.4 months. All but five patients are alive, and the actuarial overall survival for the 69 patients is 91% +/- 4% at 4 years from the start of IFN. The best indicators of relapse were the neutrophil alkaline phosphatase (NAP) score and degree of residual bone marrow hairy cells (%HCL) at the completion of therapy. Patients with NAP less than 30 (n = 21) had the best prognosis (median FFS, 30.4 months), while those with NAP greater than or equal to 30 and %HCL less than or equal to 30 (n = 21) or %HCL greater than 30 (n = 16) had intermediate and poor prognoses, respectively (median FFS, 23.5 and 12.4 months) (P = .0005). Fourteen of the relapsing patients are evaluable for response to a second course of IFN, with seven PRs and four MRs. Stratified randomized trials are indicated to determine the role of maintenance therapy for responding patients.