Nosocomial infections are those that become evident 48 h or more after a patient is admitted for treatment in a hospital or in another health-care setting. These infections cause substantial morbidity and mortality in patients who are immunosuppressed. Over the past few decades, understanding of host vulnerability has improved and more rigorous management and infection-control practices have been adopted for treating susceptible populations. Despite efforts, outbreaks continue to occur. In this Review, we outline current knowledge of the incidence and microbiology of various nosocomial infections in patients with cancer-a large, immunosuppressed population.