Nosocomial infections are a distinct reality for many hospitalized patients. However, it is possible with presently available measures to substantially reduce the threat of such infection for patients in whom some physical barrier of normal immunity has been bypassed. For these patients, conscientious attention by health care personnel to such things as careful insertion and aseptic care of urinary catheters and intravenous cannulae, and cleanliness of ventilatory equipment and other mechanical devices used in intensive care units is needed. In contrast, the potential risk of actual hospital-acquired infection in patients with impaired cellular or humoral immunity is very great. No easy solutions to prevention and effective treatment of infection in these immunosuppressed patients are available. The general approach of protection from harmful bacteria, judicious use of antibiotics when infection occurs, and immunologic reconstitution of the deficient host with specific antibodies and competent phagocytic cells seems encouraging.