Alemtuzumab (Campath-1H) is a humanized CD52 monoclonal antibody that targets normal as well as malignant B- and T-lymphocytes. Alemtuzumab has significant antitumor activity in chronic lymphocytic leukemia (B-CLL) but also induces immunosuppression. We describe a case of fatal adenovirus infection in a heavily pretreated patient with fludarabine-refractory B-CLL receiving alemtuzumab therapy, drawing attention to the fact that also viruses other than cytomegalovirus (CMV) and herpes simplex (HSV) need to be considered in B-CLL patients with fever of unknown origin while on alemtuzumab treatment.