The management of the immunocompromised host with fever presents the challenge of stabilizing the clinically ill patient and identifying the well-appearing patient who may harbor a potentially life-threatening infection, The authors critically review the prevalence, incidence, and risk factors for serious infection and recommend strategies for the emergent evaluation and treatment of patients with fever and chemotherapy-induced granulocytopenia, sickle cell anemia, or the human immunodeficiency virus. Although a patient with any of these three diseases is it particularly high risk for significant bacterial illness, the likely pathogens and natural course of disease differ, An awareness of the prevalence mid incidence of significant infection and risk factors for these infections allow the clinician to make appropriate decisions concerning treatment mid disposition.