A number of effective treatments are available for patients with hairy cell leukemia (HCL). 2-Chlorodeoxyadenosine (2-CdA) induces more than 80% complete responses, but is associated with profound suppression of CD4+ lymphocyte counts. However, the duration of each is uncertain. We have analyzed a previously reported cohort of 40 patients who had responded to 2-CdA. Eight patients (20%) have relapsed at a median of 16 months (range, 3 to 23 months). The remaining 32 patients were observed for a median of 30 months (range, 7 to 43 months). No patients have died. At 3 years, the actuarial disease-free survival rate is 77% (95% confidence interval, 70% to 84%). The median CD4+ lymphocyte count before therapy was 743/microL (range, 58 to 2,201/microL). The median CD4+ nadir after treatment was 139/microL (range, 25 to 580/microL). There was a single opportunistic infection and no second malignancies observed. Although there was evidence of some improvement in CD4+ lymphocyte counts on sequential testing, CD4+ counts remained significantly lower than baseline (P < .0001) at a median of 23 months after therapy (median, 237/microL; range, 25 to 514/microL), and were also lower than baseline (P < .002) in those patients with more than 1 year of follow-up (median, 27 months; range, 13 to 42 months). The median time to reach an absolute CD4+ lymphocyte count of 365/microL, the lower limit of the normal range, was 40 months. Although responses to 2-CdA are durable in the majority of patients with HCL, the uncertain long-term consequences of the observed CD4+ lymphocytopenia suggest caution in the broad application of this therapy.