Body substance isolation (BSI) is a system of infection precautions intended to reduce nosocomial transmission of infectious agents among patients and to reduce the risk of transmission of hepatitis B virus, human immunodeficiency virus, and other infectious agents to health care personnel. Harborview Medical Center in Seattle, Wash., was the first facility in the United States to implement the BSI system. Between 1984 and 1988 a systematic evaluation of the implementation process was conducted and the effects of BSI on appropriate glove use by hospital personnel and on the incidence of nosocomial colonization and infection by sentinel organisms was measured. Results of the evaluation showed (1) significant increments in knowledge of infection control procedures and practices as measured by comparing written examination responses before and after training sessions, (2) significant increases in appropriate glove use as determined by direct observation of hospital employees for 18 months, and (3) significant reductions in nosocomial colonization and infection caused by sentinel microorganisms during the period from 1984 to 1988.