This study attempts to characterize the response of patients with chronic lymphocytic leukemia (CLL) to the purine analog 2-chlorodeoxyadenosine (2-CdA). We have treated 10 patients with 2-CdA, at a dose 0.05-0.1 mg/kg/daily, for 7 days as a 2-hour infusion. Mean age was 54.6 years (range, 34-68 years). Mean time from diagnosis to treatment with 2-CdA was 37.0 months (range, 8-84 months). All the studied patients had received preliminary therapy consisting of other than 2-CdA chemotherapeutic regimens. Eight out of 10 patients had Rai stage III-IV disease. Four patients had Coombs positive hemolytic anemia before 2-CdA treatment. Seven patients responded to 2-CdA. Two complete remission (CR) and 5 partial remission (PR) were achieved. All patients but one with Coombs positive autoimmune positive hemolytic anemia achieve complete resolution of hemolysis. Severe neutropenia was frequent, and serious infections were noted in 20%, 43% and 50% of cases during the first, second and third course of 2-CdA, respectively. We conclude that 2-CdA is an effective agent in relapsed CLL patients, particularly in cases complicated by autoimmune hemolytic anemia.