Twenty-three adult patients with relapsed or refractory chronic lymphocytic leukemia (CLL) or prolymphocytic leukemia (PLL) were treated for up to 12 weeks with the anti-CD52 monoclonal antibody alemtuzumab. Patients were a median of six years from diagnosis and had been treated with a median of four chemotherapy regimens (median of 24 total cycles) prior to enrollment. Fourteen patients (61%) had received prior monoclonal antibody therapy with rituximab. Adverse symptoms were primarily mild to moderate fever, rigor/chills, nausea/vomiting, or fatigue/malaise in up to 86% of patients. Patients with low blood counts at the initiation of alemtuzumab tolerated therapy well. A total of 17 patients were evaluable for disease response. Nine patients (53%) responded with complete remissions in the peripheral blood. Of these nine, five were evaluated by bone marrow biopsy with four complete responses (CR) and one partial response. Six of the nine presented with nodal disease at the start of alemtuzumab therapy with three CRs and three partial responses. Alemtuzumab is a monoclonal antibody that offers effective treatment for chemotherapy refractory CLL and PLL and is generally well tolerated in the outpatient setting.