A hospital is a milieu conducive to the development and spread of infection. Patients' own flora, microorganisms in the hospital environment or those carried by medical personnel, intrinsically contaminated commercial products, and infected patients are major reservoirs of nosocomial pathogens. Use of invasive devices greatly amplifies transmission, colonization, and susceptibility to infection. Staphylococcus aureus infections are caused by autochthonous strains or exposure to medical personnel who are carriers; infections with gram-negative bacilli are endogenously acquired or derive from contact with contaminated aqueous sources within the hospital. Carriage of microorganisms on the hands of noncolonized medical personnel appears to be a major mode of cross-infection. Research is needed to better define the epidemiology of endemic nosocomial infections in order to develop more effective control measures, to evaluate more widely the efficacy of protective isolation, and to improve the safety of devices. More consistent application of infection control measures would have considerable immediate benefit.