Forty one patients with hairy cell leukemia (HCL) were treated with 2-chloro-deoxyadenosine (2-CdA) administered in various schedules. Complete remission (CR) was achieved in 31 (76%) patients and partial remission (PR) in 9 (22%). The mean duration of remission (CR + PR) was 25.2 months (range 9-45 months). One patient did not respond to therapy. Twelve out of 16 patients (75%) achieved CR after 5-day intravenous infusions of 2-CdA and 19 out of 25 patients (76%) after 7-day courses. In 19 out of 23 patients (82.6%) CR was achieved after intermittent 2-hour infusions and in 12 out of 18 (66.7%) after continuous 24-hour infusion. The differences were not statistically significant. Side effects of 2-CdA were similar in both groups except for infections, which were less frequently observed in the group treated for 5 days. The results of our study suggest that 2-CdA can be effectively administered to patients with HCL using 5-day courses and a 2-hour daily infusion.