Febrile neutropenia (FN) is a clinically important adverse effect of myelosuppressive chemotherapy. If patients present with FN, attention is focussed on well-recognized sites of origin of infection: the airways, urinary tracts, and skin. However, infections can be only documented clinically in about two-third of febrile episodes, whereas a causative microbial pathogen cannot be identified in the majority (>70%) of cases.