B-cell chronic lymphocytic leukemia has traditionally been treated with alkylating agents and purine analogues. The introduction of alemtuzumab, a CD52 monoclonal antibody with significant clinical activity in chemotherapy refractory B-cell chronic lymphocytic leukemia, is accompanied by a side effect profile different from that resulting from chemotherapy. The intravenous administration of alemtuzumab is usually accompanied by transient infusion-related side effects manifesting primarily as flu-like symptoms. These reactions can be reduced by use of corticosteroid prophylaxis, and will subside gradually during continued treatment. Alternatively, administration of alemtuzumab subcutaneously may markedly reduce the occurrence of general side effects but results in limited transient local skin reactions in most patients. Neutropenia (grade 4) may occur in approximately 20% of patients, but is usually transient and/or responds promptly to colony stimulating factor therapy; episodes of febrile neutropenia are infrequent. The major side effect of alemtuzumab is T-cell depletion, leading to an increased risk of infection, in particular reactivation of cytomegalovirus. This event typically occurs 3 to 8 weeks after initiation of therapy, coinciding with the T-cell nadir. With vigilance and early detection, these infections are usually manageable and do not cause organ failure. Preliminary data indicate that other infections following alemtuzumab therapy do not seem to occur at a frequency that is higher than expected, probably because of the general prophylactic use of cotrimoxazole (trimethoprim and sulfamethoxazole) and valacyclovir in combination with clinical tumor regressions. The overall safety profile of alemtuzumab appears to be beneficial in relation to disease severity and prognosis in patients with fludarabine-refractory B-cell chronic lymphocytic leukemia.