A prospective study of hospitalized patients was done to determine the frequency of colonization of the oropharynx with Gram-negative bacilli and the relation of such colonization to nosocomial infection. Twenty-nine per cent of the patients became colonized. Associated with colonization were antimicrobial therapy, age and clinical severity of illness. Nosocomial infections developed in 12.5 per cent of the patients, 33 per cent of whom were colonized with Gram-negative bacilli.