To assess the factors responsible for oropharyngeal colonization with gram-negative bacilli among elderly persons in institutions, we performed a cross-sectional survey of 407 volunteers, 65 years of age and older, who had not received antimicrobials in the previous four weeks. Colonization increased with level of care: from 9 per cent in independent residents of apartments to 60 per cent in patients on an acute hospital ward (P less than 0.0001). Klebsiella species was found in 41 per cent of those with colonization, Escherichia coli in 24 per cent and enterobacter species in 14 per cent. There was no association between numbers of normal flora and numbers of gram-negative bacilli. Associated with colonization were bladder incontinence, deteriorating or terminal clinical status, inability to walk or perform activities of daily living and incapacitation due to neoplastic, respiratory and cardiac disease (P less than 0.05). Multivariate analysis indicated that respiratory disease and being bedridden contributed most to colonization.