Long-term results of both pretreated and previously untreated patients (pts) with hairy cell leukemia (HCL) using uniformly a single 7-day course of 2-chlorodeoxyadenosine (2-CdA) by continuous infusion are reported. In addition, the probability of obtaining another response with this drug in pts who relapsed after 2-CdA treatment will be addressed. A total of 44 consecutive pts (34 males, 10 females) with a median age of 57 years (range 33-77) at the time of initiation of 2-CdA treatment were analyzed. In all, 11 pts were pretreated with either splenectomy (n=6), interferon alpha (n=9) or deoxycoformycin (dCF) (n=3) or all procedures in sequence. Two pts treated with dCF did not respond to dCF, but only 2-CdA. The median time to the start of 2-CdA treatment of the 11 pretreated pts was 47 months (mo) (10-160). Out of 44, 43 (98%) achieved complete response (CR) (13 pts with residual disease-RD), one pt reached a good partial response with a single cycle of 2-CdA. Out of 44 pts, 13 had no nonhematologic toxicities at all. Toxicities (WHO grade I-IV) were mainly of grade I and II, in one pt grade IV infectious complication. Bone marrow biopsies were performed at the time of recovery of hematopoiesis, thereafter at 2-3 mo intervals, thereafter at 6 mo, and finally annually in 35 pts. The median follow-up is 8.5 years (0.1-12.2). Disease-free survival from the start of 2-CdA treatment is 36% at 12 years (median 8.4 years), 17/44 pts relapsed. Nine of these pts were treated with 2-CdA again, eight achieved a second CR (median 2.5 yrs), one pt did not respond. Eight of our cohort had a second malignancy before receiving 2-CdA. Six pts died in CR due to the second malignancy. The overall survival at 12 years after the start of 2-CdA treatment is 79%. 2-CdA is a safe and effective treatment of HCL inducing complete remissions in the majority of pts with only a single cycle of 2-CdA, and a paucity of toxicities. Responses are durable and long-lasting. Pts who relapsed following treatment with 2-CdA responded to subsequent retreatment with 2-CdA.