Alemtuzumab is a humanized monoclonal antibody directed against lymphocytes through the CD-52 receptor, an antigen being found on > 95% of peripheral blood lymphocytes and monocytes, and to a smaller extent on granulocytes. It is an effective immunotherapeutic agent in patients with malignancies such as non-Hodgkin lymphoma, B cell chronic lymphocytic leukemia and T cell pro- lymphocytic leukemia. Adverse side effects are increasingly recognized in patients receiving alemtuzumab, mainly including fever, rigors, nausea/vomiting, skin rash; other severe alemtuzumab-related reactions have also been described, such as lymphopenia and neutropenia leading to both opportunistic (e.g. cytomegalovirus) and non-opportunistic infections. Digestive complications have more rarely been described, i.e.: gastroenteritis and peritonitis. We recently observed a case of particular interest as the patient with T cell prolymphocytic leukaemia treated with alemtuzumab, exhibited symptomatic reactivation of CMV infection and developed subsequently typhlitis.