This phase II pilot study determined the efficacy and safety of alemtuzumab (Campath-1H; Burroughs Wellcome, United Kingdom) in patients with chronic lymphocytic leukemia (CLL), all of whom had previously received fludarabine and other chemotherapy regimens. Twenty-four patients were treated with intravenous alemtuzumab at six centers in the United States. The target dose of 30 mg over 2 hours, three times weekly, was administered for up to 16 weeks. Responses were evaluated by an independent panel of experts using 1996 National Cancer Institute-sponsored Working Group criteria. Safety assessments included analysis of lymphocyte subpopulations. Antimicrobial prophylaxis was not mandatory. Eight patients (33%) achieved a major response (all partial remissions), with a median time to response of 3.9 months (range, 1.6 to 5.3 months). The median duration of response was 15.4 months (range, 4.6 to >or= 38.0 months), the median time to disease progression was 19.6 months (range, 7.7 to >or= 42.0 months), and the median survival time was 35.8 months (range, 8.8 to >or= 47.1 months). Acute infusion-related events, mainly grades 1 and 2, were most common and most severe in the first week. Ten patients (eight nonresponders and two responders) experienced major infections on-study. Pneumocystis carinii pneumonia was reported in two patients on-study; neither had received prophylaxis. Median CD4+ and CD8+ counts decreased and then began to increase by the end of the study, with further recovery by 1-month follow-up. One of 53 samples obtained from 10 patients had a low titer of alemtuzumab antibodies. Alemtuzumab has significant activity in poor-prognosis, fludarabine-treated CLL patients. However, because of a relatively high incidence of opportunistic infections accompanying profound lymphopenia, future protocols should include mandatory prophylaxis.