The available evidence on the efficacy and safety of alemtuzumab in patients with CLL is weak and the results are inconclusive. We found three uncontrolled open studies involving patients with resistant or active CLL. One study involved patients with disease that was refractory to both alkylating agents and fludarabine, while another employed alemtuzumab as first line therapy. A third study looked involved a combination of alemtuzumab with rituximab another monoclonal antibody. These limited data suggested that alemtuzumab may be efficacious in CLL. However, the studies raised questions about safety. In all the studies, a proportion of patients had a complete or partial response to alemtuzumab therapy. However, serious and even fatal adverse effects were common. Without good quality data from controlled studies, it is difficult to establish the absolute effects of alemtuzumab in people with CLL. Randomised controlled studies, comparing alemtuzumab with the gold standard established treatments (alkylating agents or fludarabine) or placebo, are needed before the effects of alemtuzumab can be reliably defined.