Factors that promote oropharyngeal colonization of seriously ill patients with gram-negative bacilli are as yet poorly understood. In this investigation, 34 subjects who required intensive care were studied; 18 (53%) were colonized with gram-negative bacilli. Oropharyngeal epithelial cells of all colonized patients contained adherent bacilli. Fewer alpha-hemolytic streptococci but greater numbers of Pseudomonas aeruginosa and Klebsiella pneumoniae (P less than or equal to 0.01) adhered in vitro to buccal epithelial cells from colonized patients than to cells from noncolonized patients. Adherence of bacilli to buccal cells was inhibited in vitro by concanavalin A but not by bovine serum albumin or phytohemagglutinin. Brief exposure of buccal cells to trypsin increased adherence of bacilli. Prior adherence of one species of bacilli inhibited subsequent adherence of a second species. These findings suggested that epithelial cells of the upper respiratory tract contain binding sites for gram-negative bacilli. Factors associated with serious illness appear to increase the availability of these binding sites, thus facilitating colonization of the upper respiratory tract with gram-negative bacilli.