Patients with peripheral T-cell lymphomas (PTLs) have an extremely poor prognosis when relapsed or refractory to conventional chemotherapy. We have studied alemtuzumab, a humanized anti-CD52 monoclonal antibody, as therapy for patients with heavily pretreated and refractory PTL. Fourteen patients entered the study. All had clinical stage III or IV disease. Patients received a rapidly escalating dosage of alemtuzumab during the first week and, thereafter, 30 mg intravenously 3 times per week for a maximum of 12 weeks. Trimethoprim/sulphamethoxazole and valaciclovir prophylaxis was given to all patients. The overall response rate was 36% (5 of 14). Three patients achieved a complete remission (CR) and 2 patients a partial remission. The durations of the CRs were 2, 6, and 12 months, respectively. Toxicity included cytomegalovirus reactivation in 6 patients, which was successfully treated with ganciclovir or foscarnet; pulmonary aspergillosis in 2 patients; and pancytopenia in 4 patients. Epstein-Barr virus-related hemophagocytosis was observed in 2 patients. Five patients died of causes related to the treatment, in combination with advanced disease. We conclude that alemtuzumab is active when used in patients with advanced, heavily pretreated PTL, although it is associated with significant hematologic toxicity and infectious complications. Further studies are warranted in younger patients and patients with less advanced disease.