Morbidity and mortality in patients with malignancies are increased by viral infections. These mostly are reactivations of asymptomatic latent infections. They primarily concern clinical entities associated with the reactivation of herpes viruses, such as varicella zoster virus (VZV) and cytomegalovirus (CMV). Respiratory tract infections caused by influenza, parainfluenza or respiratory syncytial virus (RSV) are less common. Since reactivation of latent infections has major clinical impact, antiviral prophylaxis is an attractive approach for patients expecting immunosuppression. The main risk factor for clinically relevant reactivation is profound disruption of cellular immune response. Duration and severity of chemotherapy induced neutropenia are of lesser importance. The risk of viral complications rises significantly in the presence of sustained suppression of T-cell function, e.g. in recipients of allogeneic stem cell transplants or of alemtuzumab (Campath-1H) antibody therapy. The objective of this guideline is to review the basis of prophylactic strategies and to provide recommendations for clinicians treating patients with haematological malignancies and solid tumors.