Nutritional therapy of patients receiving mechanical ventilation includes enteral feeding. To determine the frequency of gastric microbial colonization in patients receiving enteral nutrition, 18 patients with acute respiratory failure receiving ventilation were studied. Multiple sources of tracheal colonization were then evaluated to determine the frequency of tracheal transmission of gastric flora and the relationship of gastric colonization and transmission to nosocomial respiratory infection. The stomach was colonized in every patient who received enteral feeding. Tracheal colonization occurred in 89 percent (16 of 18) of patients. In 12 of these 16 patients, 14 organisms colonized the trachea after transmission from the stomach and/or oropharynx. Thirty-six percent (five of 14) of organisms transmitted to the trachea originated from the stomach, whereas another 36 percent (five of 14) were first recovered from the oropharynx. Four tracheally transmitted organisms were first recovered simultaneously from the stomach and oropharynx. Nosocomial respiratory infection developed in 11 patients (63 percent). Three infections occurred during the six-day study period, one of which was associated with transmission of a gastric organism. Multiple sources of tracheal colonization occur in patients receiving enteral nutrition. The stomach is an important source of tracheal colonization. Enteral nutrition can be associated with gastric flora colonizing the trachea and causing nosocomial respiratory infection.