A comparative, prospective study was made of the incidence of infection in the lower airway (purulent tracheobronchitis and pneumonia) in long-term patients who were mechanically ventilated due to respiratory failure of noninfectious origin. Twenty-eight patients were randomly allocated into a study group (A, n = 13) in which a nonabsorbable paste containing 2% tobramycin, 2% amphotericin B, and 2% polymyxin E was administered locally to decontaminate the oropharynx, and a control group (B, n = 15) in which a paste without antibiotics was also applied to the oropharynx. We studied the effectiveness of the prophylactic technique in decontaminating the oropharynx and trachea of organisms potentially pathogenic for the respiratory system. Decontamination was successful in ten of 13 patients in group A vs. one of 15 patients in group B (p less than .001). The results demonstrated a lower rate of infection in the lower respiratory tract in the study group (three patients with tracheobronchitis and no pneumonias) than in the control group (three patients with tracheobronchitis and 11 with pneumonia), the difference between both being highly significant (p less than .001). Two (15%) patients in group B developed sepsis of pulmonary origin. None of the patients on prophylactic treatment developed this complication. Although the overall mortality was similar in both groups (group A, 30% vs. group B, 33%), we believe that infection contributed to a great extent to the death of two of five patients in group B. We conclude that nosocomial pneumonia, which is a frequent complication in critically ill patients on mechanical ventilation, could be prevented by local application of nonabsorbable antibiotics to the oropharynx.