A cross-sectional prevalence study of gram-negative bacillary oropharyngeal colonization in the geriatric population with single oropharyngeal cultures was performed on three different groups of individuals, the first consisting of healthy individuals, living independently, the second residing in a nursing facility and the third hospitalized. A longitudinal incidence study by serial weekly cultures was in addition conducted on a fourth group of hospitalized individuals. In the cross-sectional study, gram-negative bacilli were isolated from 23% of the hospitalized group, whereas only 7-10% of the other two groups were colonized (p less than 0.05). In the longitudinal study, the colonization was intermittent and transient, being most prevalent on admission, i.e. 23%, but gradually decreasing during the hospital stay to 7% after ten weeks. Only hospitalization and prior treatment with antimicrobials increased prevalence of colonization. On only one occasion was a lower respiratory infection predated by oropharyngeal colonization with the same organism, indicating that such colonization may not be a risk factor for the development of pneumonia in the elderly.