The oropharynx, stomach, and trachea are all potential reservoirs for gram-negative organisms in mechanically ventilated patients. The pathogenic importance of each site in respiratory infection may differ between mechanically ventilated patients who are medically stable and the critically ill, and these differences may be important in understanding the pathogenesis of nosocomial infection. We prospectively studied seven patients requiring chronic ventilatory assistance who were otherwise medically stable to determine the pattern of gram-negative colonization of these three sites. Serial weekly oropharyngeal, gastric, and tracheal cultures were taken over a 6-mo period in our Respiratory Care Unit for chronically ventilator-dependent patients. Pseudomonas aeruginosa (PA) was present more frequently and persistently in the trachea than the oropharynx and stomach (p < 0.01) and members of the family Enterobacteriaceae (Ent. species) were also observed more commonly in the trachea than the oropharynx (p < 0.01). PA was seen in 6.7% of gastric specimens whereas Ent. species were found in 40% of gastric specimens. Six identical strains from a total of 53 gastric isolates and 128 oropharyngeal isolates were cultured coincidentally from these two sites. Coincidental isolation of 11 strains was observed in 177 tracheal isolates and 53 gastric samples. Documented transfers from stomach to oropharynx ascertained by sequential isolation occurred in one of 118 cultures and transfer from stomach to trachea occurred in three of 134 cultures.(ABSTRACT TRUNCATED AT 250 WORDS)