The Centers for Disease Control (CDC) has developed a new set of definitions for surveillance of nosocomial infections. The new definitions combine specific clinical findings with results of laboratory and other tests that include recent advances in diagnostic technology; they are formulated as algorithms. For certain infections in which the clinical or laboratory manifestations are different in neonates and infants than in older persons, specific criteria are included. The definitions include criteria for common nosocomial infections as well as infections that occur infrequently but have serious consequences. The definitions were introduced into hospitals participating in the CDC National Nosocomial Infections Surveillance System (NNIS) in 1987 and were modified based on comments from infection control personnel in NNIS hospitals and others involved in surveillance, prevention, and control of nosocomial infections. The definitions were implemented for surveillance of nosocomial infections in NNIS hospitals in January 1988 and are the current CDC definitions for nosocomial infections. Other hospitals may wish to adopt or modify them for use in their nosocomial infections surveillance programs.